Nightcrawlers (Lumbricus terrestris) are a popular bait for bluegills, bass, trout, catfish, carp and perch. Scouring toughens up nightcrawlers so that they stay on the hook better and can be cast farther distances. Many fishermen scour nightcrawlers before using them for bait.
Place the nightcrawlers
A plastic quart yogurt container is a good spot to put the worms and the bedding. Punch slits in the lid so that air can get in the container, but don't make the slits very wide, so that not much moisture will evaporate.
After several days the worms will have ejected all of the castings (digested dirt) in their intestinal tract into the bedding. They will be slightly smaller in size. Their skin will glisten. They will be tougher and stay on a hook longer than before. They will be scoured.
The classic bedding for scouring is damp sphagnum moss. Fresh green sphagnum moss is best, if you can obtain it where you live. Sphagnum moss is also sold commercially as peat moss
Use fresh grass clippings if you have trouble obtaining sphagnum moss. This is my favorite bedding to use for scouring a small number of nightcrawlers, because it is easy to obtain and works well. Look for an unmowed patch of lawn and tear off the clippings when you need them. Don't used browned or dried clippings that have been in the sun. The grass clippings must be damp, but not so damp that water collects in the bottom of the container, which will kill the worms. Usually if there is dew on the grass when you harvest it, that is enough moisture. Otherwise, wet your hands and sprinkle the grass with water. Place the nightcrawlers on top of the grass clippings and close the container. They will work their way down into the grass clippings.
Nightcrawlers will stay alive for a week or two in scouring bedding, although there is no food in the bedding material (except for a little bit in commercial peat moss) so they lose weight and are less valuable as bait. The best thing to do is only scour a dozen or two nightcrawlers at a time, when you know that you are going to use them as bait.
Store-bought nightcrawlers will usually already be scoured, depending on the bedding they are sold in. Open the container before buying the nightcrawlers, and make sure that they are lively and firm. If they are lively and soft, buy them anyway and scour them. If they are not lively, don't buy them.