Opportunities for Parasitic environments

I'm trying to look at sites and situations in which a parasitic object could be used. I'm finding it a help to find the hosts and then think of the parasite. My ideas are very abstract and not very clear but the idea of the stair slide is quite nice visualy. Then looking at existing parasitic forms the plug which I could put forward into unusual designs of objects using a socket not necessarily for electricity. Then bizarre sofa storage which has been done a million times for nasty things like remote controls and magazines. I also thought about inclosing space on the sofa making it more of a den or experience. Finally an idea for a parasite which stops the cold tap from turning on too far I suppose saving water but this would probably just be an annoyance.