1+1=5 and Other Unlikely Additions, by David LaRochelle, ill. by Brenda Sexton (Sterling, 2010.)
Can you think of why 1+1 would = 5? Using clues from the previous page, kids guess why 1+1 equals various unusual sums. The answer is explained at the page turn.
This is a fun book that incorporates addition...and algebra! When you think about it, the two ones really represent x and y. I did this with my family; we were able to guess "sum," not all. As we got into the mindset of looking for clues on the page to solve the equation, we got better at it. This would be a fun book to read with your class, and then come up with equations of your own.
On his Web site, www.davidlarochelle.net, the author offers a teaching guide describing ways of doing this. It also includes printable activities such as The Secret Life of Numbers, in which kids turn numbers into drawings.
So, why does 1+1=5? You'll have to read the book to find out!