Light

Light
Only light can travel at the speed of light, Nothing else.
And what is it exactly? Do we really know? 
We contend it is of two natures - particle and wave.
Does that help?
It might be fun to consider what it does, and then think upon what it might be that has such strange properties.
For example, we all know light passes through a pane of glass.
However, do we know how it does that?
One might say it passes between the molecules of the glass.
That would mean if the glass were thicker, less light would emerge on the other side. 
It turns out that if the glass is made thicker, let's say by adding a second pane, more light may pass through. Add another pane, and even more light might come through. This effect continues until some magic thickness is achieved, after which the amount of light making its way through decreases to some minimum. Add more glass and more light gets through. Pretty strange, no?
Let's make it simpler.
Light is reflected off a mirror.
If you shine it directly on the surface of a mirror, it gets reflected right back at you. 
News flash.
It's not actually reflected.
A photon hits an electron orbiting an atom of the mirror. That electron is excited into a higher, more energetic orbit. Then it descends back to its original orbit, releasing energy in the form of a photon. All well and good.
But how does that photon know where to go?
​Light. The final frontier.