To solder a copper pipe, the water must be drained. (unless you have access to a pipe freezer) The quickest way to drain down a system, is to open up all of the faucets. You will remember, that when the top of a drinking straw is closed off with a thumb, that the soda will not drain out. Sometimes the pipe to be soldered is in a "low" spot. If this is the case, cutting and holding the tubing down will help to drain it. A plumbers trick on hard to drain pipe is to jamb a bit of dry bread in the line to plug off the water. Remember, that there must be an open faucet when soldering. If not, the last joint soldered will leak.