Mum has a cat who has this particular (and peculiar) habit of drinking from the faucet whenever he's had treats.

It's pretty hilarious how well he's trained the humans. Whenever he's had a treat, he'll walk toward the bathroom and flick his tail, ears turned back, expectantly waiting for someone to follow. If no one follows, he'll continue on his way and jump up on the counter top. If he still hasn't collected anyone's attention, he'll emit a distinctive (and very squeaky!) meow that he uses for no other purpose than "I want a drink" and will do so periodically until someone comes to turn the faucet on. Otherwise he'll lay there until he's noticed (if not, he'll give up and finally go to one of the water bowls).

He's also finicky about the flow rate. If it's turned up too high (he prefers a gentle stream—almost a dribble) he'll stare at you until the flow rate is corrected.

Talk about training his humans indeed!