Energy comes in many forms: mechanical, chemical, and gravitational just to name a few. One form of energy that humans have lots of experience with is thermal energy. We experience thermal energy as temperature, and we can feel it! Hot and cold and everything between. Temperature is how we express thermal energy. If there's a lot of thermal energy, we call that hot. If there's not much, we call that cold.

How do we know if something is hot or cold? For most things, we have to touch them. This is because the nerves in our hands feel the world around them -- they rely on conduction to transfer thermal energy from a hot mug (for example) into our skin for our nerves to interpret.

To figure out exactly how much thermal energy something has (that is, what the temperature is), we rely on thermometers. Let's say our thermometer is made of glass. Glass is a material, and we know that all materials are made of molecules. When we stick the glass thermometer into a mug of tea, the glass molecules touch the tea molecules (many different kinds of molecules in tea!). They "feel" each other. Molecules vibrate and collide with each other. When molecules collide, they're transferring energy -- just like when you're bowling, you are transferring energy from the ball to the pins (hopefully!)

If you have a warm and cold cup of water, the molecules in the warm cup are moving faster, that is, they have more thermal energy. To move faster, the molecules need more space. So the hotter water is, the more space it takes up (except for freezing, water is weird). A glass thermometer is filled with red alcohol that gets bigger when hot (expands), and shrinks when cold (contracts).

So we fill a tiny (but long) glass tube with red alcohol, seal it, and stick it in our tea. What happens?

Tea molecules transfer thermal energy to the glass molecules by conduction
Glass molecules transfer energy to the red alcohol (also by conduction)
The alcohol gets bigger and fills up more of the tube (expansion)
We read a marking on the outside of the tube and see that our tea is at the perfect drinking temperature of 142 °F (61 °C)

Congratulations, you've just measured thermal energy! Enjoy your drink :)