.. Beer shortage

Did you look at the taps?

Yes, and the pipes on the wall must have something to do with it, clearly. Look at them.

There is one for water and one for beer, again.

You could mess with the pipes.

With telekinetic powers, of course. They will be switched.

Say to the bartender that Gil would need another drink.

Hmm... maybe you could get rid of all of the beer.

Did you examine the fountain outside, already? Where is it getting its water?

Look at the pipe near the ground.

The water is coming from the tavern pumps!

Do you see where we are going here?

So, get back in and switch the pipes again. Don't tell the bartender!

Now, pump all the beer away from the tavern using the fountain lever.