Don't be so gullible

A phrase used by bullies and jocks, so that they can easily trick someone because they trust them.

Bully: Hey! Your shoe is untied!
Nerd: Wait, what?!
Bully: *kicks the nerd in the balls*
Nerd: *chipmunk voice* OUCH! MY BALLS!!!
Bully and his friends: BWAHAHAHAHAHAHAHAHAHAHAHA!!!!!!
Bully: Don't be so gullible, you dumbass!!!