As Halloween is right around the corner, here's a video from ByteSizeScience on how hard candy is made:
Some things of note:
(1) Hard candy is technically a glass made of sugar!
(2) There are three stages of sugar boiling, and the maximum temperature that it reaches determines the physical properties of the resulting candy (I think this is primarily a function of how much water is remaining in the sugar mixture).
(3) If you watch the video, you'll figure out why hard candy always has a ring around the edge. This was something I didn't know.