Most 3D pens work by feeding plastic through the pen, which then melts it before it comes out of a nozzle at the end of the pen (that’s what we call “extrusion”). The pen heats up to a different temperature depending on the material you put inside the pen.
So how on earth can I then draw in 3D? When it leaves the pen, the melted plastic is soft and malleable, but within a few seconds it solidifies, creating a solid, stable, pick-up-able structure.