What do you say to the kid who asks "What will I ever use this for?"

Well it appears that none other than Euclid had a response for that question. Simon Singh, in Fermat's Enigma, tells a story about Euclid when he was teaching, as the first head of the mathematics department at Alexandria:

"Euclid searched for mathematical truth for its own sake and did not look for applications in his work. One story tells of a student who questioned him about the use of the mathematics he was learning. Upon completing the lesson, Euclid turned to his slave and said, "Give the boy a penny since he desires to profit from all that he learns." The student was then expelled."

If only...

Euclid of Alexandria