Condoms.

A guy goes into a drugstore to buy condoms.
"What size?" asks the clerk.
"Gee, I don't know."
"Go see Sophie in aisle 4." He goes over to see Sophie, who grabs him in the crotch, and yells, "Medium!" The guy is mortified! He hurries over to pay and leaves quickly.
Another guy comes in to buy condoms, and gets sent to Sophie in aisle 4. Sophie grabs him and yells, "Large!" The guy struts over to the register, pays, and leaves.
A high school kid comes in to buy condoms.
"What size?" The kid embarassedly says "I've never done this before. I don't know what size." The clerk sends him over to Sophie in aisle 4. She grabs him and yells "Clean up in aisle 4!"