Casting silicone is very expensive, so over the years Grant Thompson has tried to find a cheaper alternative to making LEGO candy molds.
He started off trying silicone caulk from the hardware store, but globs of it would take weeks and weeks to finally firm up. That wasn’t acceptable because he needed something that would harden in 24 hours or less. About 3 years ago he came across an Instructable by Mikey77 showing that mixing corn starch into the silicone caulk could be used to make it firm up in under an hour. Grant did a number of experiments on his own and eventually found out that corn starch wasn’t actually needed at all, but that all you needed was to add a little food coloring.
He still likes to use corn starch though because it helps prevent the caulk from sticking to his hands as he works with it. The stuff is really hard to get off if it sticks to skin!