It's always interesting to see which rats take to the wheel and which don't.  You'd think it would be a genetic trait, but it doesn't really seem to be that way.  I've had lots of 'wheelers' whose mommy and daddy never gave it a look.  All in all, my experience has been that about half the litter will take to the wheel.

Those are the ones that live.

Or have a better chance of living.  The sad truth is, snakes have a right to eat, too.  As such, there's always a chance that a few of them are going to end up as snake food.

Fair is fair.

The kids are exposed to the wheel almost from the start, and, as I said, some take to it and some don't.

The day before I take them to the pet store, I watch which ones use the wheel.  When one does, I yank it out and draw a ring around the base of its tail with a felt tip pen.  I then put it in another cage and continue to see who else takes to the wheel.  After a few hours, I've got them separated.  Since most people who buy a rat or mouse hope it'll run on the wheel in their cage, it seems only fitting that the 'wheelers' are the ones the pet store owners keep aside as pets, and the rest have to face whatever fate has in store for them.