Decide how many kiddos you want participating at a time. Place a bowl of water in the middle of the table. Set one penny and one eye dropper for each child around the bowl. Ask each kiddo to guess how many drops of water they think they can get to onto the face of the penny.
Have the youngins' fill the eye dropper and carefully drop one drop at a time onto the penny. Ask them to count the drops as they go. Keep counting until the penny can no longer hold the water and rolls off the sides. Was the number more or less than they thought. Ask them why they think that is the case.