Chain falling off

[fillsley]

i bought a bike for my son it has 6 gears and is a mountain bike the chain came off and how ever many times i put it on as soon as i pedal it makes a really bad noise and comes off. someone help please im a bike virgin and dont know anything about them. thanks

[nfmisso]

need pictures or even better video; even then it is hard to diagnose without a much more detailed description.

[painkiller]

is coming off the front ring(s) or the back? check for side play in the crank, look for bent or damaged links in the chain, look for bent or damaged cogs and chainrings