This is going to sound rather odd....

That IS weird. It's probably just because they're shiny, and shaped in a way that makes them convenient to put in their mouth (almost like a bottle-top..Sort of). Who knows. Kids are weird.

I have found that kids like all things that aren't toys. If its something they can pick up or see its going in their mouths. It's enough to drive you nuts but it happens. I believe its more so curiosity that gets to them. Batteries are small enough for them to pick up, they are shiny as said in the previous response (well most are), and its something that isn't a toy. I would suggest doing what the other mom on here suggested as far as taping the back of the mouse so he has a harder time to get into it to get the batteries. Since I'm sure if he is smart enough to know how to take it apart to get the batteries to begin with he will eventually master the removal of the tape as well. Maybe find a higher place or somewhere you could keep the mouse to where he isn't able to get to it as easily. I guess this is why toy companies are getting so serious about screwing the "doors" up on the toys where the batteries are enclosed. I think thats a gripe of mine since becoming a mom is the amount of time it takes to not only open their toys up from the store by fighting with the wire ties, screws, etc. but then after getting the toy out of the package you have to screw off the cover to put batteries into it. Goodluck with your little one and I'm sure he will grow out of his battery obsession. If not it could be a great sign of what he may do in the future! :)

I would tape the back of the mouse and anything else that has a battery in it, He could coach on the battery, they are acidic and should not be digested.