MEK is the common name for the organic solvent Methyl Ethyl Ketone. It is the "cement" used to bond together pieces of plastic, as it will soften most of them; when it evaporates the soft plastic edges fuse and you have "cemented them together". It's not supposed to soften plastic resin after it has cured, but the hardening process is never 100% complete.

http://www.tapplastics.com/product/mold_making_materials/casting_products/tap_clear_lite_casting_resin/75