Tom picks up a yellow phone book that turns green the moment he touches it.
The first time Jerry drinks almost all of the poisoned milk, but the second time he gets to the plate, it's full again.
When Jerry drinks the "toxic" milk, which Tom put outside the mouse hole, there's only a little bit left in the bowl. Then later when Jerry goes back to drink another dose of the milk, the bowl has more milk in it than it did after Jerry drank it the first time.
Jerry is shown drinking Tom's milk thru a straw but the amount of milk in the bowl never changes or goes down.
Jerry never sees what poisons and other chemicals Tom puts in the milk so he could not duplicate the formula without knowing what Tom put in originally.