Katy Perry sings "Hot N Cold" with Elmo on Sesame Street!

grover

http://www.sesamestreet.org/muppets/groverGrover, also known as Grover Monster, is a Muppet character on the popular television show Sesame Street. Self-described as lovable, cute and furry, he is a monster who almost never uses contractions when speaking or singing.
Grover was originally performed by Frank Oz, who provided the character's voice regularly from his earliest appearances until 2001. Eric Jacobson began performing Grover in 1998; he has performed the character regularly since 2002, although Oz still performs him occasionally. Grover's birthday is October 14

elmo

http://www.sesamestreet.org/muppets/elmoElmo is a Muppet character on the children's television show Sesame Street. He is a furry red monster with a falsetto voice, who hosts the last full fifteen minute segment on Sesame Street, Elmo's World, which is aimed at toddlers. He was most recently puppeteered by Kevin Clash, until Clash resigned in late 2012; going forward, the role of Elmo will be played by understudies who were trained by Clash.

cookie monster

http://www.sesamestreet.org/muppets/cookie-monstercookie monster is a Muppet on the children's television show Sesame Street. He is best known for his voracious appetite and his famous eating phrases: "Me want cookie!", "Me eat cookie!", and "Om nom nom nom" (said through a mouth full of food). He often eats anything and everything, including danishes, donuts, lettuce, apples, bananas, as well as normally inedible objects. However, as his name suggests, his preferred food is cookies. Chocolate chip cookies are his favorite kind; oatmeal cookies are his second favorite.