Hi all
i have a 21 speed mongoose moutain bike i had it for about a year now and all of a sudden when rideing my bike to work.i had noticed that my chain keeps on skiping!.does anyone have any ideas? it is very difficult to peddle when its like thats.

Your cable has probably "stretched" ... and, the consequence is that the indexing is now off.

So, you need to EITHER use the barrel adjuster which is probably on the shifter to take out the slack when the chain is on the smallest cog OR to loosen the rear derailleur's anchor bolt (again, when the chain is on the smallest cog), tighten the cable to remove the slack by pulling on it, and then tighten the rear derailleur's anchor bolt while maintaining the tension on the cable.

The skill level needed is very low for either, but it can be botched ... so, you may have to re-do it more than one time to get it right.

okay thaks