Remember when you were a kid and wadded up white bread into little balls before eating it like popcorn? A whole slice of bread compressed down to nearly nothing, revealing the essential truth of white bread: it's mostly air. Plumbing types can use this principle when soldering replacement sections of pipe into place. Often, moisture in the old pipe causes problems, steaming through the bond.
To fix that, stuff white bread (strip the crusts) into the pipe. It stops moisture from reaching the joint while you're soldering. Then it conveniently dissolves into small water-logged crumbs inside the pipe, exiting harmlessly through a faucet or into the sewer system.
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