water

All cats do this. This is why in my house, we have a constant 'toilet lid down' policy since we pour cleaner in there.

The reason why cats do it is because they prefer COLD water over the water in their bowl, which usually sits at room temperature. Once a cat discovers a source of cold water he will keep going back to that no matter how gross a source it is. This can be, but is not limited to kitchen sinks, bathroom sinks, bathtub spigot, and of course toilets.

The best thing to do is buy one of those cat fountains that constantly recycle and chill the water. Start with a cheap one in case your cat rejects it, if he loves it you can upgrade to a fancy cat fountain and your cat will be in kitty heaven.