system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does water help a suction cup work better?
gpt
Suction cups work by being pressed against a flat surface. While being pressed, the air underneath is pushed out. So you've created a negative pressure area as the cup "wants" to return to its original shape, but can't. So this all hinges upon not letting air back under the cup. So you need a good seal. Water essentially fills in the "gaps" or imperfections around the suction cup, thus giving a much better seal.

On a similar note, you will find suction cups work poorly if at all on uneven surfaces, surfaces with holes, etc.