itsagaz said: Another possible option would be to get a couple of cans of shaving cream, soak them in a sinkful of hot water for about 20 minutes, then atttach the nozzle of each can to a three-foot (or so) length of flexible aquarium tubing - whatever size is appropriate to fit tightly over the nozzle of the can. (The nozzle/button assembly on most shaving cream cans is a standardized size and removable, so you might want to glue the tubing onto the nozzle and wrap it with tape to insure a tight, leak-free fit.)
Put the other end of each tube into one of the participants' underwear. When the button on the can is pushed, the recipient's underwear will suddenly begin to fill with shaving cream. It's quite an interesting feeling.

Of course, you need to either push the button yourself, have a third person push it, or find a way to automate the process, but it's worth the fuss.

bsmithyst said: Although this sounds interesting, will shaving cream not start to sting after a bit of friction?

I'm liking the idea of some kind of device that sends a manageable amount of goop (like 500ml) into each other's pants ... just needs to be relatively easy to make, and something that won't sting a we slide against each other!

Gregster said: However you manage to achieve it, please write it up as a story and post it for us to read. Love your stories!