What's the deal with elastics?

Ew. I reached for an elastic today from my elastic-drawer, and got my hand in something sticky and greasy. It turned out that the elastic I touched was sort of half-melted and really weird!

When I went through them, it turned out a lot of the elastics had become really weird: Some were dry and crinkled up and broke when I pulled them, and some all slimy and melted-like.

In the end, I threw away like one third of them!

Have you guys tried something like that before? Why is this happening?