cracked(pun intended)

After covering the basic street drugs and their specific effects, one bright classmate raises her hand to ask,
"Prof., but what's that drug where you tie a rubberband around your arm and then stick a needle in?"
"What?! It's any drug that you inject!"
"No, that I know. But what's that drug where you tie a rubberband and then you stick a needle in???"

uh, ye, hokey.