We just learned about the Light Year.
If you are standing in the water in the ocean or a big lake,
you can watch a wave going by you, something like this:
![water wave](https://upload.wikimedia.org/wikipedia/commons/b/be/Shallow_water_wave.gif)
(from: wikipedia - wave)
If you stood in one place and watched a lot of waves going by, it might look something like this:
![harmonic wave](https://upload.wikimedia.org/wikipedia/commons/7/74/Simple_harmonic_motion_animation.gif)
And if you took a picture of a wave while it was standing still it might look like this:
(from: wikipedia sine)
If you measured the distance from the start of the up wave, to the end of the down wave that is called the wavelength.