Gerbil is biting on metal...what should I do?

In my Zoology cl, me and my partner are taking care of two gerbils (Ben and Jerry). For the past few weeks, both gerbils has been biting on a lot of things, such as my partners hands, and the metal wheel that's in the cage (not plastic). I'm just a bit curious why both of them are acting this way...and please I would like to know soon.