This is one of the funniest videos about mathematics I’ve seen: Arthur Benjamin, a professor of Mathematics at Harvey Mudd College, shows how to do mental math extremely quickly:

Can you mentally multiply enormous numbers faster than a calculator? Memorize a 100 digit number? Figure the day of the week of anyone’s birthday? These are just some of the many feats of mind that will be demonstrated at this event. Mathematics can be a difficult and perhaps puzzling subject for many. But watch in amazement as one of the world’s fastest human calculators shows his ability to turn math into magic.

Arthur Benjamin, Ph.D. is a professor of Mathematics at Harvey Mudd College in Claremont, CA. He is the author of Secrets of Mental Math and Proofs that Really Count as well as co-editor of the journal Math Horizons. Fusing his love of both mathematics and magic, he has appeared at the World Science Festival in New York and the Magic Castle in Hollywood. Reader’s Digest calls him “America’s Best Math Whiz”.