It's about this time of year that you start to hear Philadelphia native Will Smith's summer anthem on the radio.  'Summertime' is an essential tune for a day lounging by the pool or on the beach.  Smith recently stopped by 'The Late Show with David Letterman' and ended up singing a bit of the classic rap.

As Smith walked onto the stage before his interview, the band was playing an instrumental version of 'Summertime.'  Smith started rapping out loud and was given a mic to finish out the song.

It's pretty cool!

Watch it here!