On the engine of a motorcycle, the cylinder head is made of a thick piece of clear Plexiglas that allows to observe the arc generated by the spark plug. It causes inflammation of the gas mixture in the combustion chamber. For this, it must be able to generate thousands of electric arcs per minute while withstanding the heat and pressure generated by the explosion inside the cylinder. A good way to see inside a combustion engine but not for too long, as you can see the Plexiglas can't go at a very high temperature so it melts and the spark plug fall.