A Pet You Cannot See

I told my daughter's class (to whom I am teaching Spanish) that as part of our last day, Tuesday, I would bring them a "pet that moved and jumped but that you cannot see." They are all so excited that one girl even offered to give me three dollars if I would reveal the secret and tell her what it was. Another girl offered me three million dollars; I would've told her, but I didn't think she could come up with the money! : ) So, the kids may be jumping like Mexican jumping beans if they don't arrive by the first of the week, but they can wait another week if need be.

Thanks so much for your help, and I'll let you know when they get here!

Regards,
A.B.