This is one I haven't seen before: our cat (about 18 months old, rescued from the street before 6 months old) always wants to be sure there is food floating in her water bowl before she drinks from it. If the water is clear, then without fail she very intentionally switches to her food bowl, takes 3-4 pieces in her mouth, and goes back to her water bowl to drop them in. (She ignores this ritual with other water sources, such as the toilet when someone forgets to leave the lid down...)
The pieces of food gradually soak up water, deteriorate, and make it rather unappealling — from a human point of view. What's the appeal to her? To clarify, we're not concerned, just curious.
One hypothesis so far is that she just likes the taste of her food and wants to flavour her water. But I gather that taste doesn't play a huge role in a cat's diet. Another one is that she isn't sure about the water level and wants some "buoys" floating in it to better gauge it. If so, she's a pretty weird cat, considering they don't all do this!
What are your ideas?