Denise's birthday brain teaser was written by Dr. Joseph Yeo Boon Wooi of Singapore's National Institute of Education. Yeo is also the author of the viral "Cheryl's birthday" problem, and this puzzle is very similar to the original... but it's even more complicated: Denise wants her friends to guess her birthday, and gives them 20 possible dates. The dates for year 2001 are Feb. 17, Mar. 13, Apr. 13, May 15, and Jun. 17. The dates for year 2002 are Mar. 16, Apr. 15, May 14, Jun. 12, and Aug. 16. The dates for year 2003 are Jan. 13, Feb. 16, Mar. 14, Apr. 11, and Jul. 16. The dates for year 2004 are Jan. 19, Feb. 18, may 19, Jul. 14, and Aug. 18. Albert knows the month, Bernard knows the day, and Cheryl knows the year. Albert says, "I don't know her birthday, but I know Bernard doesn't know." Bernard says, "I don't know her birthday, but I know Cheryl doesn't know." Cheryl says, "I don't know her birthday, but I know Albert doesn't know." Albert says, "Now I know her birthday." When is Denise's birthday? Stumped? Watch the video to get the answer.