Strange Change Machine
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The Strange Change Machine was a Mattel toy from the late 1960s, in which "shape memory" plastic figures of prehistoric animals and so forth could be reconstituted from compressed "time capsule" form, and re-compressed back into that form. It consted of a domed heating chamber, with a heating element at the bottom, and a hand-cranked screw-press for re-compressing the figures into nondescript blocks ("time capsules").
To reconstitute a figure, one would first pre-heat the "capsule" on the side of the machine, then toss it into the heating chamber, and close the door. Over the course of a few minutes, the plastic would soften enough for the original shape of the figure to reassert itself, and it would slowly open up into that shape. (Tongs were provided for removing the still-hot figure.)
To compress a figure back into "time capsule" form, one would place the figure into the heating chamber, until it began to sag slightly, then remove it with the tongs, stuff it into the open press, and crank the press closed. After a few minutes, the compressed figure would cool enough to retain its compressed shape.
[edit] How it worked
The process is actually quite similar to the way heat shrink tubing works: in the case of the tubing, it is molded in its "shrunk" size, cross-linked (usually by exposure to certain types of radiation, such as electron beams) to convert it to a thermoset, then re-heated and stretched. In the case of Strange Change figures, the figure is molded (in what appears to have been polyethylene), exposed to electron beams to cross-link it, coated with a long-lasting release agent to keep it from sticking to itself when compressed, then (in much the same process described above, only with power equipment) compressed into "time capsule" form.
