Thyroid hormone tells all of the cells in your body how busy they should be. Too much thyroid hormone (hypERthyroidism), and your body goes into overdrive; not enough thyroid hormone (hypOthyroidism), and your body slows down.
Place the pot on the stove and turn the heat to high. Check for steam escaping from under the lid, then lift the lid carefully to see how the water is doing. Look at the water. If large bubbles are rising from the bottom of the pot to the surface, the water is boiling.
While water boils, it's the same process, but (!!) particles jump of into air in the entire liquids volume. Which means that even on bottom of your liquid, you can find particles that have so high energy that they jump into gas state (that's why you see bubbles while water is boiling).