The body is made from ABS plastic while the core is made from a special alloy containing graphite. The graphite and metal alloy work together to produce a line when friction is applied and due to the special properties of the core, the Metacil is capable of writing a 16km long straight line without sharpening. The tip is quite fragile though so we would recommend just using it with paper to avoid any issues!
 
This Light Knock version is a retractable variation of the original, so you can click the tip away - in some ways it's more like a ballpoint than a mechanical pencil. The tip is made to last a very long time, so you don't need to keep clicking out more lead, just replace the core when it eventually wears down too much.
 
There are replacement tips for them, of course, and they're available in a range of barrel colours.
10mm diameter, 145mm long.