Why does my cat wanna pee in the sink???

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All my cats are fixed. Daisy the 3 yr old has recently stopped pooping outside the box. It might be the combination of the two litters she likes. As far as Foxy going in the kitchen sink, last night I put a empty dish pan in one side of my double sink (where she goes) and a litter pan in the other side. Lord behold! She pee and pooped in the litter. Hopefully she will continue to use the litter. After awhile I will try putting the litter on the floor near the sink. If that works I will gradually move it elsewhere.

Thanks for your suggestions about sanitazing the area. I've tried using Only Natural Pet Power and next I'll try Vet's Best Flea/Tick spray product I ordered online. The flea powder I use on the rugs has Silicon Dioxide (Diatomaceous earth) 88 % and Piperonyl Butoxide 1.0% and Pyrethrins .2%. It makes alot of dust that you should not breath in. Don't want the animals to breath it in either. I left it on for 2 days then vaccumed. I know I should do it until I see no more fleas but don't like powder on the rugs or dust in the air. I'm going to try the spray next. It has 2.7 5 Sodium Lauryl Sulfate, .7% Clove Extract and .3% Pepperment Oil. It says to use on rugs, beds and even on pet. I will try the beds and rugs only. It suppose to Kill Fleas, Flea Egs, Ticks and Mosquitos.

Oh yes, I have six cats and 6 litter pans. One is in the screen porch. I have a one level home. I empty them as soon as they poop. Before bedtime and first thing in the morning I empty anything in them. I use the lumping kind so what is left is not usually soiled. When I recently changed litter I did sanitize the pans.

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I think one reason may be the fleas are tormenting them. Before I put Revolution and Capstar on them they wold not play on the floor with any toys. They would run from table top to sofa and try to avoid the floor. Maybe this is why Foxy does not want to use the litter which is on the floor. All the cats seem to avoid the floor but she and Daisy would goe outside the box in the sink or tub. I have been trying a few products on the rugs and have washed their beds but they still don't like the floor.
My other cat still use the litter boxes. I had Daisy and Foxy checked out for cystitus and they are OK, no infections.

I still get a flea or two on me while watching TV. They are still around. They will eventually be extinct since I vaccum every day and the cats have revolution which kills the fleas and the eggs.