“I’m too cold. Get me a sweater!” “This soup’s too hot!” “Phew, I’m sweating.” “Yowtch, that pan handle burned me!” If you’ve ever made any of the above comments, then you were talking about thermal energy. Very clever of you, don’t you think?   Thermal energy is basically the energy of the molecules moving inside something. The faster the molecules are moving, the more thermal energy that something has. The slower they are moving, the less thermal energy that something has.

I’m sure at some point you’ve said, “Wow, my internal thermal energy is way high! I need a liquid with a low thermal energy.” What…you’ve never said that?! Oh, wait. I bet it sounded like this when you said it, “Wow, I’m hot! I need a cool drink.” Whenever we talk about the temperature of something we are talking about its thermal energy. Let’s get started by watching this video:
Objects whose molecules are moving very quickly are said to have high thermal energy or high temperature. The higher the temperature, the faster the molecules are moving. You may remember that temperature is just a speedometer for molecules.

You may have asked yourself the question, “So, if everything is made of molecules, and these molecules are often speeding up and slowing down…what happens to the stuff these molecules are are made of if they change speed a lot? Will my kitchen table start vibrating across the room if the table somehow gets too hot?” No, it’s pretty unlikely that your table will begin jumping around the room, no matter how hot it gets. However, some interesting things do happen when molecules change speeds.

You can get started by watching this video, and afterward either read more about it or start your experiments!

Scientific Concepts:

  • The terms hot, cold, warm etc. describe what physicists call thermal energy.
  • Thermal energy is how much the molecules are moving inside an object.
  • The faster molecules move, the more thermal energy that object has.
  • There are three different scales for measuring temperature. Fahrenheit, Celsius and Kelvin.
  • Temperature is basically a speedometer for molecules. The faster they are wiggling and jiggling, the higher the temperature and the higher the thermal energy that object has.
  • Your skin, mouth and tongue are antennas which can sense thermal energy.
  • There are four states of matter: Solid, liquid, gas and plasma.
  • Solids have strong, stiff bonds between molecules that hold the molecules in place.
  • Liquids have loose, stringy bonds between molecules that hold molecules together but allow them some flexibility.
  • Gasses have no bonds between the molecules.
  • Plasma is similar to gas but the molecules are very highly energized.
  • Materials change from one state to another depending on the temperature and these bonds.
  • Changing from a solid to a liquid is called melting.
  • Changing from a liquid to a gas is called boiling, evaporating, or vaporizing.
  • Changing from a gas to a liquid is called condensation.
  • Changing from a liquid to a solid is called freezing.
  • All materials have given points at which they change from state to state.
  • Melting point is the temperature at which a material changes from solid to liquid.
  • Boiling point is the temperature at which a material changes from liquid to gas.
  • Condensation point is the temperature at which a material changes from gas to liquid.
  • Freezing point is the temperature at which a material changes from liquid to gas.


Select a Lesson

Special Science Teleclass: Thermodynamics
This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too! You’ll discover how to boil water at room temperature, heat up ice to freeze it, make a fire water balloon, and build a real …
Sensing Temperature
Have you ever wondered how an ice-cold glass of water gets waterdrops on the outside of the cup? Where does that water come from? Does it ease it’s way through the glass? Did someone come by and squirt the glass with water? No of course not.
Balloon Gymnastics
Is it warmer upstairs or downstairs? If you’re thinking warm air rises, then it’s got to be upstairs, right? If you’ve ever stood on a ladder inside your house and compared it to the temperature under the table, you’ve probably felt a difference. So why is it cold on the mountain and warm in the …
Homemade Thermostat
If you can remember thermostats before they went ‘digital’, then you may know about bi-metallic strips – a piece of material made from of two strips of different metals which expand at different rates as they are heated (usually steel and copper). The result is that the flat strip bends one way if heated, and …
Ghost Coin
This spooky idea takes almost no time, requires a dime and a bottle, and has the potential for creating quite a stir in your next magic show.  The idea is basically this: when you place a coin on a bottle, it starts dancing around. But there’s more to this trick than meets the scientist’s eye. …
Food Dye Currents
When something feels hot to you, the molecules in that something are moving very fast. When something feels cool to you, the molecules in that object aren’t moving quite so fast. Believe it or not, your body perceives how fast molecules are moving by how hot or cold something feels. Your body has a variety …
Making Clouds
Indoor Rain Clouds Making indoor rain clouds demonstrates the idea of temperature, the measure of how hot or cold something is. Here’s how to do it: Take two clear glasses that fit snugly together when stacked. (Cylindrical glasses with straight sides work well.) Fill one glass half-full with ice water and the other half-full with …
Boiling Room Temperature Water
The triple point is where a molecule can be in all three states of matter at the exact same time, all in equilibrium. Imagine having a glass of liquid water happily together with both ice cubes and steam bubbles inside, forever! The ice would never melt, the liquid water would remain the same temperature, and …
Stairstep Candles
Fire is a chemical reaction (combustion) involving hot gases and plasma. The three things you need for a flame are oxygen, fuel, and a spark. When the fuel (gaseous wax) and oxygen (from the air) combine in a flame, one of the gases produced is carbon dioxide. Most people think of carbon dioxide as dry …
Liquid Crystals
If you’ve completed the Soaking Up Rays experiment, you might still be a bit baffled as to why there’s a difference between black and white. Here’s a great way to actually “see” radiation by using liquid crystal thermal sheets. You’ll need to find a liquid crystal sheet that has a temperature range near body temperature …
Football Ice Cream
Is it hot where you live in the summer? What if I gave you a recipe for making ice cream that doesn’t require an expensive ice cream maker, hours of churning, and can be made to any flavor you can dream up? (Even dairy-free if needed?) If you’ve got a backyard full of busy kids …