One of my girls, commenting on how a classmates hair was secured atop her head; "Why are your buns always so perfect?" The obvious response, "Lots of time in the gym" provoked total (if momentary) loss of control of the class.
Science teacher: Please wear your safety goggles. Student: Why? Science teacher: So you're not blinded by science.
My Junior year English Teacher: "In some muslim countries, the punishment for stealing is to cut off the hand of the offender." Me: "What's the punishment for premarital sex?"