I once lived with a cat who would check cups for liquid. 

If they were empty, she left them. If they had water, she knocked them over.

This cat is less discerning:

http://pandawhale.com/post/19224/fuck-this-fuck-that-fuck-this-thing-in-particular-cat-gif

A cat walks into a bar.

http://pandawhale.com/post/58115/a-cat-walks-into-a-bar-and-orders-a-glass-of-water-please-bartender-asks-are-you-going-to-drink-it-or-just-knock-it-over-on-purpose