When you need to cut a shape inside a piece of wood, a plunge cut is pretty much your only choice. Typically, you might use a drill with a small bit to bore a hole that you can drop your jigsaw blade ...

Cutting plexiglass with a utility knife works best for making straight cuts in thin sheets of plexiglass (1/8 inch or less). But while a utility knife is convenient (you probably have one in your tool ...