Given the solidness and weight of the toy, Stretch Armstrong wasn't exactly an action figure, or at least didn't fit in with other action figures as far as being able to stand on its own, articulate different poses, or to be used in 'fights' between figures, as boys will do.
As many kids found out - often by experimentation - the toy was filled with a thick, dark-colored gel that would ooze out of any damaged area, quickly turning hard and rendering the toy useless. The toy's soft 'skin' allowed it to be punctured easily, and of course many kids would stretch the toy beyond its normal capacity, damaging it.
Stretch Armstrong's fictional nemesis was the Stretch Monster, a similar toy with a green, scaly body and monstrous face, who featured the same stretching properties. Kenner would also release two more such figures, Stretch X-Ray and Stretch Octopus (the latter for smaller children).