So I was trying to solder an end cap on this vertical copper pipe but the main valve does not completely seal the line, so drops of water kept running into the end during blowtorching, making the job literally impossible without a specific and expensive plumber's kit. So I broke off a hunk of white bread and stuffed it into the pipe to stanch the water so I could do the job. The bread will dissolve eventually, not clogging the line, but also this is an end anyway. It worked! I don't remember ever learning of this trick, but I don’t think I came up with it, either. I must have heard it someplace and filed it away long ago.