Or just a little water in the pipe blasting out some steam with a little flux right in the eye,
That's where the expression the best thing since sliced bread comes from.
When you have to sweat a joint with a pipe that has water in it take a little white bread and ball it up and shove it in the pipe. It will stop the water from getting into the joint while you sweat it. You must be quick.
When you're done take the screen off the faucet and the bread will work it's way out.
For those who understand no explanation is needed, ...For those who don't none will.