On topic because it can be used in aid of metalworking......

I found some stuff called "Polymorph" at Maplin's, an electronics
store chain in the UK a bit like Radio Shack but better. Googling
polymorph revealed a US source for it.

This is neat ****, Maynard! It's a 500g jar of little pellets that
soften if poured into hot water -- 62C or more. The pellets turn
clear and kinda gob together like salmon eggs. It can then be
molded like modelling clay. It's a thermoplastic, can be re-melted
and re-used. They claim that it's as strong as nylon when cool.
That may be a bit of a reach, but it's definitely pretty tough stuff.
I've made no measurements, but my "feel" is that it's more like nylon
than polyethylene when cool. It can be machined (once cooled)
with sharp cutting tools -- saw, drill, mill, burr, etc. It doesn't
power-sand well because it gets gummy with heat of sanding.

A 500g jar was £11.99, about $21 US. Not too bad since it can be
re-used.

Metalworking potential: jigging of irregularly-shaped parts for
milling or drilling, particularly if said part has a nice finish that
one might want to protect. Mold the stuff around the part,
position the part and snug the vise, wait for it to cool, tighten
the vise and check/tweak positioning. When machining is done:
dunk the lot in hot water, peel off the stuff and return it to the
jar for re-use if it didn't get contaminated with cutting fluid etc.