I nuked some vinegar in the microwave. I poured a centimeter (half an inch) or so of vinegar in a bowl and cooked it in the microwave for three minutes. This was enough to have the vinegar boiling in the microwave for a little while. I then wiped out the microwave with a damp cloth. Worked wonders. Just as good as the much more expensive commercial options you can buy (for example, the Easy-Off Heat-Activated Microwave Wipes that I had bought in the past.