I don’t use them for keeping the flour out of my hair, or even my hair out of the flour. No, they are very handy for covering rising dough, either in a bowl or in a baking tin. They’re quicker and easier to use than cling film, and less wasteful, as you can use them over and over again.
They will stretch to quite a size: I can easily cover my biggest mixing bowl, which is about 35cm (14”) across. And they are surprisingly robust; my three have been going for months without showing any signs of tears or loose elastic.
Unless they get very mucky, you shouldn’t need to wash them. Just leave them out for a while to dry, turning them inside out if a lot of condensation has built up inside. Any natural yeasts that they might transfer to the dough will be killed off by baking, and might even add interesting overtones to it. However, for this reason I wouldn’t advise using them to cover anything that isn’t going to be well cooked.
I bought three for £1.07 from Boots. They do sell them on line, but delivery is three times the cost of the shower caps themselves! But either way, they are an excellent investment.
I first saw this idea on The Fabulous Baker Brothers. Since then shower caps seem to be popping up everywhere in bread books and web sites!