[bryan]

A nice simple little demo you can do is to rub a coin in your hand to warm it up, then press it against a wall or door (painted or waxed works best) hold it for a few seconds then let go and it will remain held to the wall. The simple explanation is that a layer of warm air is trapped between the coin and the wall, when the air cools it contracts creating low pressure and the higher air pressure outside the coin holds it to the wall.

Can anyone help me expand this explanation? I want to be able to draw a complete force diagram for this, i'm struggling to understand exactly what force is pushing the coin up and counteracting gravity.
Cheers

[daveshorts]

The simple explanation is friction. The the air inside the gap reduces in pressure, so air pressure outside pushes the coin against the wall. If the coin is pushed against the wall, then friction will resist any forces on the coin by gravity.