Riker has reached the stage where he wants pretty much everything mom and dad have.
If we are eating something, he will stare at us with his mouth open.
If we are drinking something, he moves his little lips like a fish.
So when TJ placed his cup of juice in front of Riker, he just had to have it!
Much to Riker's dismay, he was never successful in his attempts to reach the cup.
Riker can roll over, but he chooses not to.
See this post here.
He's no dummy.