We used an old toaster oven outside, for ventilation purposes. Melting plastic stinks and is quite toxic.
You can see the bag of acrylic beads below. There are a lot of them. They were a gift to my daughter several years ago, that just never got used.
We used the tray that came with the toaster oven, lined it with parchment paper, then filled three metal cookie cutters 1/4 of the way full with the beads. The blue and white bunny is my daughter's. She was a little more meticulous about the selection of her beads.
Set the oven on 450, and bake beads for 20 minutes. Do not do this indoors, unless you like to inhale deadly fumes.
When done discuss the results. As my kids could tell the melting of the beads from their original form into the second form was definitely a chemical reaction. However, we had a great deal of discussion, and finally agreed that when the melted beads were removed from the oven and allowed to cool, that was a physical reaction because they were still the same makeup of the melted beads, but went from liquid to solid. We will drill holes in these and make sun catchers.