A common problem when doing plumbing is how to solder a wet pipe, this can be a real problem especially if you cut into a downstairs copper pipe as the rest of the pipes in the house can take hours to drain fully. Obviously the pipe needs to be dry before you attempt to solder it, so you can actually stop the drip by placing a rolled up piece of bread up the pipe. The bread will swell on contact with the water and eventually it will stop the water from dripping out of the pipe. Once the pipe has been soldered you can remove the bread by flushing out the pipe (the pipe needs flushing out after soldering anyway).