In the microwave: Wrap the bread in a damp (not soaking) towel, place it on a microwave-safe dish, and microwave on high for 10 seconds. Check and repeat if necessary.
In both cases, keep a close eye things so the bread doesn't overcook and the towel doesn't catch fire!
Do you have any other tips for softening hard bread?
Related: Food Science: Why Bread Stales
(Image: Emily Ho)