Water trickling in copper pipe can make soldering a joint almost impossible because you can't get the fitting hot enough. Stuffing a wad of bread into the pipe will absorb the water and you can get done before it dissolves. But use white bread; whole grains can get caught in valves. Before turning water back on, temporarily remove faucet aerator.
Don’t throw away that sawdust. Keep some in a coffee can to throw on workshop spills. It’s also great for throwing on icy sidewalks in the winter.