Kitty becomes cat burglar

[jembie]

I found out from talking to my neighbour that my oldest cat goes into her house every night to eat her cats' biscuits. He also likes to climb their activity centre and recently left a mouse.
I am horrified! he gets at least 1 1/2 tins of food a day from me but the worst is that he recently was treated for Cystitis and he shouldn't be eating biscuits at all. I am quite upset that he took her a mouse too as he hasn't brought me one in a long time.

My neighbour has to leave her window open for her own cats to come and go although she says she will shut the food in another room so he can't get at it I don't know what to do from my point of view to stop him wanting to go in there. I don't really want to stop him going out at night either as he sleeps all day and gets really noisy if we don't let him out.