Ask if they've ever seen a conductor direct a symphony or orchestra, with his tuxedo, and gloves and baton.  Explain how a choir or band always follows the conductor to see when to go fast, slow, loud, soft explain the different hand motions you will be using for loud, soft, fast, slow.  Tell them they are a choir and you are the conductor and they must always follow you carefully.  Pick a song they know very well and they like.  Have fun conducting it while they sing without the piano, changing all the dynamics throughout the song, hooooooldiiiiiiiiiiing notes at unexpected places, until you direct them to continue, and speeding through other parts; going soft - loud -soft.  Exaggerate your movements.  Even the boys who normally don't like to sing enjoyed holding notes longer than usual.  Next time, give them a chance to direct the song with liberty to change tempo at will - (I had one boy have us hold a note so long that we all ran out of breath!)