<P> Shrinky Dinks are a children's toy and activity kit consisting of large flexible sheets which, when heated in an oven, shrink to small hard plates without altering their color or shape . They reached the height of their popularity in the 1980s . Most sets are pre-printed with outline images of popular children's characters or other subjects, which are then colored in before baking . </P> <P> Shrinky Dinks were invented in 1973 by two housewives (Betty Morris and Kate Bloomberg) of Brookfield, Wisconsin, as a Cub Scout project with their sons . The first kits were sold at a local shopping mall and became very popular . Shrinky Dinks were soon licensed to be manufactured by the major toy companies of the time such as Milton Bradley, Colorforms, Western Publishing and Skyline Toys . The shrink plastic is still available from many retailers and can be used for anything from charms to pins . </P> <P> The base material consists of thin, flexible polystyrene plastic (#6) sheets . Prior to heating, the plastic sheets can be colored with felt - tip pens, acrylic paint, colored pencils, etc. and cut into shapes . However, oily or waxy substances (such as cheap colored pencils, crayons, or oil paint) are not suitable because they melt or burn in high heat . When heated with the Easy - Bake Oven, a conventional oven, or a heat gun, the plastic shrinks by about five - eighths and becomes thicker and more rigid, while retaining the colored design . </P> <P> Although Shrinky Dinks are primarily an arts and crafts product marketed for children, many adult crafters and artists find the product to be suitable for jewelry making and other projects . Blank sheets are available in bulk for this purpose, and Shrinky Dinks have, rather unexpectedly, become a serious artistic medium . </P>

What kind of plastic are shrinky-dinks made of