@Antonio Diavolo - are you talking about the glass where the milk disappears as you pour it into a paper cone or where the level of the milk visibly drops in the glass (e.g. when the spectator sips through a straw that is not even near the glass)?
If it is the first one, it is easily found based on
@DavidL11229 names. I suspect that you are looking for the second one. I don't know the exact name of the prop. The only one that I can find that is commercially marketed is the Automatic Milk Glass -- but it gets terrible reviews. However, it is easy to make one - PM me if you need instructions.