The No-Magic Card Trick

Adam Weiner explains the physics behind an excellent party stunt. Hint: the wizardry's in the water

Has the law of gravity suddenly taken a vacation? Au contraire. Is the trickster in this video using sleight of hand? Nope. Can you use this trick to pick up women at cocktail parties? Possibly. Let's analyze just what's going on here.

There's a fairly substantial weight of water in the glass pushing directly down on the card. So what keeps the card (and water) from falling to the ground? The only possible answer is that there must be an upward force acting on the card, thus suspending it in place. But what is the nature of that force?

Let's recall a little fluid mechanics. Molecules are capable of exerting electrostatic forces on each other. In the case of like molecules these are called cohesive forces. For example water molecules exert cohesive forces on neighboring water molecules. Meanwhile unlike molecules (for example water molecules and playing card molecules) exert adhesive forces. In the video it appears that both adhesion and cohesion are in play. The adhesion between the water and the card (and the water and the glass) keeps everything "glued" together while the cohesion between water molecules prevents the water from "breaking" even when some fluid starts to leak out of the bottom of the glass. What's surprising is how strong these forces can be. I tried this trick using a completely full 550 milliliter bottle of water. That's around 1.2 pounds of water. The card held. However, make sure you use a glass with a relatively wide and flat rim or the forces won't be sufficient to impress your friends, neighbors and potential dates.