Gestures are special animations which players can activate during co-op gameplay. Gestures are contained as separate items in the 'Gesture' slot. There are eight default gestures for both Atlas and P-body, and 8 purchasable gestures.
Performing a gesture during the co-op campaign will at times cause GLaDOS to make a satirical comment upon how much you resemble 'humans'.
Performing gestures also act as criteria for certain achievements.