Fill a microwave-safe bowl with 2 cups of water. Add 2 tbsp. of baking soda and stir it in with a spoon.
Place the bowl in your microwave. Cook on high heat for one and a half minutes. Allow it to sit for another two minutes in the microwave. The steam will condense on the microwave ceiling and walls and help loosen grease and food particles.
Remove the bowl carefully from the microwave. Rub a clean cloth over the ceiling of the microwave to remove the grease.
Wet another cloth under your sink faucet in warm water. Sprinkle the cloth with baking soda.
Scrub the ceiling and walls of the microwave with this cloth to remove any remaining grease or food. If you find it is still difficult to get the grease off, add 1 tsp. of liquid dish detergent to the cloth.
Dampen another clean cloth and wipe it over the microwave to rinse it.