"if this is a cup, and if this is a cup, is this a cup?" riddle?

[jojoandhiggins]

my friend has this riddle where she makes a sign with one hand and says, "if this is a cup," and she makes a sign with her other hand and says, "and if this is a cup," and then she makes another sign with her hand and asks, "is this a cup?" she refuses to tell the secret to the riddle and i'm dying to know. does anybody know the secret to this riddle?

[katey1801]

i don't get it
now im dieing to know, too

[MiniKilla]

its kinda hard to figure it out considering we cant see the signs your friend is making