What are waves?

Have you ever ridden a wave in the ocean? Ocean waves travel on the surface of the water. You can see them and you can feel them. As you swim through the water, you can even make your own waves.
Have you ever seen a flag on a windy day? The wind creates waves in the flag. Both the waves in the flag and the ocean waves are waves that you can see. There are other kinds of waves. We cannot see these waves, but we experience them every day. These waves are called electromagnetic waves.

Sound is also a type of wave that we cannot see. Like ocean waves, sound waves need a medium to travel through. Sound can travel through air because air is made of molecules. These molecules carry the sound waves by bumping into each other, like Dominoes knocking each other over. Sound can travel through anything made of molecules - even water! There is no sound in space because there are no molecules there to transmit the sound waves. Electromagnetic waves are unlike sound waves because they do not need molecules to travel. This means that electromagnetic waves can travel through air and solid materials - but they can also travel through empty space. This is why astronauts on spacewalks use radios to communicate. Radio waves are a type of electromagnetic wave.