Cheryl's birthday problem is a logic puzzle that requires no math. Here is the problem: Cheryl wants Albert and Bernard to guess her birthday, and gives these possible dates: May 15, May 16, May 19, June 17, June 18, July 14, July 16, August 14, August 15, August 17. Albert knows the correct month, and Bernard knows the correct day. Albert says, "I don't know Cheryl's birthday, and I know Bernard doesn't know." Bernard says, "I didn't know at first, but I now know Cheryl's birthday." Albert says, "Now I also know!" When is Cheryl's birthday?