In a “minute to win it” game, cards are placed between cups to stack them. The contestant then pulls the card out in hopes that they will end with a set of stacked cups. Why does this work?

There is no friction between the card and the cup.

Inertia will keep the top cup from moving very far. Gravity will pull one cup on top of another.

The opening of the cup is so much bigger than the bottom, even though the cup moves they will both be stacked together.

Both cups are moved the same amount by the friction between them and the card.