Nick Jonas serenades a pentagon and octagon with a slinky R&B song on Saturday's episode of 'Sesame Street.' Nick Jonas stopped by Sesame Street to teach kids different kinds of shapes. In a preview of his guest spot on former PBS program that has recently moved to HBO, Jonas joins Cookie Monster, the Count and Bert to serenade a pentagon as they "Check That Shape." Jonas will appear in an episode set to air on Saturday, May 21st on HBO at 9 a.m.