Why Suction Cups Stick (It's Not Suction)
written by Stefan Christoph
A quick note on where this comes from: this series was born at a reception at a broadcast conference in Amsterdam. Octopus was on the menu, and an octopus arm had stuck itself to someone’s plate so firmly that trying to pick up the arm lifted the plate along with it. That started a conversation about what “suction” actually is, and it turns out the honest answer is “not suction.” Everything below is standard physics; sources are at the bottom, and if I’ve oversimplified something, the comments are the point.
The air around you is heavy
You don’t notice it, but you’re standing at the bottom of an ocean. It’s just made of air, and it’s roughly a hundred kilometres deep. The weight of that column presses on every surface it touches at standard atmospheric pressure, about 101,325 pascals [1]. Pascals are abstract, so here’s the version that lands: that’s about 10 newtons on every square centimetre, from every direction at once. Ten newtons is roughly the weight of a one-kilogram bag of sugar, resting on a patch the size of your thumbnail.
You don’t feel crushed because the push is equal on all sides, and your body pushes back from the inside. Balance everywhere means no net force. Remember that word, balance, because breaking it on purpose is the whole trick.
Press the cup
Here’s the thing to play with. Press the cup against the glass, then peel it off, and watch where the force comes from.
Notice what the arrows are doing. Before you press, the air pushes the cup equally from above and below, so there’s no grip. After you press and the cup springs back, the arrows from above win, because there’s much less air underneath pushing back up. Nothing reached out and pulled. The outside simply stopped being cancelled.
What actually happens
The sequence has four beats.
First, you press the cup. Squashing the flexible dome squeezes most of the air out from underneath it, and the soft rim seals against the smooth surface on the way down.
Then the cup springs back. Rubber wants to return to its dome shape, so the volume under the cup grows again, but almost no air is left inside to fill it. The same handful of molecules now rattle around a bigger space. Fewer collisions on the walls means lower pressure. That is Boyle’s law in one sentence [2]: squeeze the same gas into a bigger volume and the pressure falls.
Now the atmosphere wins. Outside, the full weight of the air is still pushing down at about 10 N/cm². Inside, the pressure has dropped well below that. The two no longer cancel, and the leftover, the difference, is a net force pressing the cup flat against the wall. Say it plainly: the vacuum isn’t pulling the cup in. The atmosphere is pushing it on.
Finally, peeling releases it. Lift the rim a little and you open a gap into the low-pressure pocket. Air rushes in, the inside climbs back up to atmospheric, the difference collapses to zero, and the cup drops into your hand. That same leak, happening slowly through an imperfect seal, is why a cup you stuck up last week is on the floor this morning.
The math is refreshingly small
The holding force is nothing more than the pressure difference times the area you sealed:
F = (P_outside − P_inside) × A
Take a small cup, four centimetres across. Its area is about 12.6 cm², which is 0.00126 m². Give it a firm press that clears out around 80% of the air, dropping the inside to roughly 20,000 Pa. The difference is about 81,000 Pa, so:
F ≈ 81,000 Pa × 0.00126 m² ≈ 102 N ≈ 10 kg
Ten kilograms of hold from a cup you can bend with two fingers. In practice you get less: real seals leak, the load usually arrives at the rim as a peel rather than a straight pull, and few of us press out a full 80%. That gap between the tidy 10 kg and the “rated up to 3 kg” on the packet is honesty about imperfect seals, not bad physics.
The same trick, elsewhere
Once you see “lower the pressure and let the atmosphere push,” you start finding it everywhere:
- Drinking through a straw. You don’t pull the drink up. Your mouth enlarges, the pressure in the straw drops, and the atmosphere pressing on the drink’s surface pushes it up to you. This is also why straws have a hard limit: even a perfect vacuum could only push water up about 10 metres, because that’s all the lifting the atmosphere can pay for [4].
- Vacuum cleaners. The fan lowers the pressure inside; the room’s air pushes itself, crumbs and all, into the nozzle. “Slightly-lower-pressure cleaner” would be the honest name. It wouldn’t sell.
- Breathing. Your diaphragm enlarges the chest cavity, the pressure in your lungs dips just below ambient, and the atmosphere pushes air in [5]. You’ve been running this exact machine your whole life, about sixteen times a minute.
- The plunger. A suction cup with a handle and a job.
False friends
And then there are the things that look like the same phenomenon but run on entirely different physics. Same costume, different actor:
- Sticky tape and gecko feet. No pressure difference involved at all. Those stick by molecular attraction (van der Waals forces) between surfaces in extremely close contact [6]. The tell: a gecko would stick just fine in a vacuum chamber, while a suction cup gives up instantly.
- Two wet glass plates that won’t come apart. Mostly the water itself: capillary and viscous forces hold the plates, with a bit of our pressure trick joining in when you pull them straight apart. A hybrid, not a twin.
- Airplane wings. The tempting one. A wing also lives on a pressure difference, so isn’t it just a suction cup doing 900 km/h? No. The cup’s pressure difference is static: seal a pocket, remove air, done. A wing’s is dynamic: it exists only while air flows over the wing, and the popular “air on top has a longer path so it must go faster” explanation is wrong anyway, as NASA patiently keeps explaining [7]. Same currency, completely different way of earning it. Lift deserves its own lunch break, and it’ll get one.
Fun consequences
Once you see it as pressure difference, a pile of everyday observations line up:
| Observation | Why |
|---|---|
| Won’t stick to a rough or textured wall | Micro-channels under the rim leak air in; the inside can never drop below outside, so ΔP stays near zero. |
| Sticks better when slightly wet | Water fills the microscopic gaps and improves the seal, and unlike air, it can’t spring back to fill the cavity. |
| Would be useless in space | No atmosphere means nothing pushing from outside. ΔP ≈ 0, no grip, no matter how hard you press. |
| Falls off after a few days | Air leaks back in through the imperfect seal, the inside pressure creeps up, and the difference slowly bleeds away. |
| An octopus does it underwater | Same physics, different fluid, and a clever upgrade. |
That last row is where I started, so let me close the loop.
Back to the octopus
An octopus sucker is the biological version of the same machine, and arguably a better one. Instead of relying on a springy dome to enlarge the cavity, the animal uses muscle to actively pull the centre of the sucker outward once the rim has sealed [3]. That drops the pressure inside the little chamber below the surrounding water pressure, and the water outside (which, several metres down, pushes even harder than our atmosphere does) presses the sucker onto whatever it’s holding. Same idea as the rubber cup on your bathroom tile: seal a pocket, lower the pressure inside, and let the fluid outside do the gripping. The arm on that dinner plate wasn’t clinging. The room was pushing it down.
Which is a strangely nice thing to know the next time a hook refuses to stay on the wall. It isn’t failing to suck. The air just found a way back in.
Next Tuesday on Lunch Break Physics: your fridge doesn’t make cold, it moves heat. Same “it’s not what it looks like” spirit, with another thing you can poke at.
Sources
- [1] Standard atmospheric pressure and the pascal — 101,325 Pa ≈ 10.1 N/cm².
- [2] Boyle’s law — pressure and volume of a fixed amount of gas are inversely related.
- [3] Octopus suckers and pressure-based attachment — muscular reduction of internal pressure below ambient water pressure.
- [4] Suction and the ~10 m water-column limit — atmospheric pressure supports at most about 10.3 m of water.
- [5] Mechanics of breathing — inhalation driven by sub-atmospheric pressure in the expanded thoracic cavity.
- [6] Gecko adhesion via van der Waals forces — dry adhesion from molecular attraction, no pressure differential.
- [7] NASA: the “Equal Transit Time” lift theory is incorrect — the pressure difference is real, but the longer-path explanation is not.
About the Author
Stefan Christoph is a Principal Solutions Architect at AWS, focused on agentic AI, media & entertainment, and helping builders move from demo to production. He writes about AI architecture, developer productivity, and the future of software.
This is a personal blog. Opinions expressed here are my own and do not represent the views or positions of my employer.
❤️ Created with the support of AI (Kiro)