a sheet of paper is withdrawn from under a glass of milk without spilling it if the paper is removed quickly. this best demostrates that?

The physics principle that best explains this demonstration is inertia.

Inertia is the tendency of an object to resist changes in its state of motion. When an object is at rest, it tends to stay at rest unless acted upon by an external force. Similarly, when an object is in motion, it tends to stay in motion with the same speed and direction unless acted upon by an external force.

When you quickly remove the sheet of paper from under the glass of milk, you are applying a force to the paper that exceeds the force of the milk's surface tension holding it in place. The paper moves rapidly, but the milk stays in its original position due to its inertia. The milk resists any change in its state of motion, so it remains relatively still as the paper is taken away.

It's crucial to note that this demonstration works best when performed swiftly to minimize the time during which forces can act on the milk. Slowly removing the paper would give the milk more time to respond to the force and be disturbed, leading to spillage.