Yank on the string, and the bottom block comes out from the stack without disturbing the other blocks.

This demonstration is similar to the tablecloth demonstration (12.12) and the screw-over-flask demonstration (12.18), and the principle behind them all is identical. If you pull the block slowly, friction between it and the pile of blocks on top of it can keep the pile together as you pull. If you yank quickly on the string, however, you can accelerate the bottom block so quickly as to overcome friction. When this happens, during the time it takes you to pull the block out from under the stack, friction cannot accelerate the stack enough to move it any significant distance. The stack thus stays put. As in the other demonstrations, you are able to do this because of the inertia of the blocks above the one you are pulling, and this inertia is proportional to the mass of the blocks. (That is, the force required to accelerate the blocks at a given rate is proportional to their mass. Of course, so is the friction force that you have to overcome.)