If you are try to solder, or sweat, copper pipe, it's important that all of the water be out of the pipe. If there is water in the pipe, the water will absorb the heat and it can be very hard, even impossible, to heat the pipes up to the point where the solder will flow and you will get a good joint. There are some cases though where you can't get all of the water out of the pipes to be soldered; either there is residual water stuck in the pipe, or there is a slow steady flow of water that can't be stopped.

In these cases, a trick you can use is bread. White bread, with no crusts. Tear the bread apart into pieces and out it into the pipe to soak up the water, or to stop the flow of the water. This will give you a short period of time where the pipes are water-free so they can be soldered more easily. The bread will dissolve quickly and get flushed out of the pipes, so there is no damage to your water system.