The waves cause the water molecules in your food to vibrate, which causes friction, and thus produces heat.
(The waves; cause; the water molecules in your food to vibrate, which causes friction, and thus produces heat) (the water molecules in your food to vibrate; causes; friction) (the water molecules in your food to vibrate; thus produces; heat)
The waves the water molecules in your food to vibrate the water molecules in your food to vibrate
cause causes thus produces
the water molecules in your food to vibrate, which causes friction, and thus produces heat
friction
heat