Here's what to do: Mix one-half cup of water with one-half cup of white distilled vinegar in a microwave-safe bowl. (Use at least a four-cup bowl, otherwise you risk your water boiling over.) Microwave on high for several minutes, until the water comes to a rolling boil and the microwave window steams up. Let it cool for a few minutes before opening the door, then wipe the loosened food debris with a damp sponge and you're done.
For a more pleasant scent, instead of using vinegar add a few slices of orange, lemon or lime to one cup of water and microwave until boiling.