You Drink With Your...?

A moment ago, I brought Big M to bed. He has been playing computer games for around half an hour before going to bed. Once we reached the bedroom, he complaint his back side was painful

Me: Why you never complaint while you are playing games just now?

Big M: Only now i feel it. Today I got past motion. That is why my backside pain.

Me: Everyday already give you drink vitagen, backside still pain pain. No need to drink vitagen lah...

Big M: Why cannot drink vitagen wor... you drink with your mouth ma... not your backside...

Me: ?? :)