system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does heat shrink tubing work?

One of the most fundamental properties of most materials in the universe is that they expand when heated.  Why is heat-shrink tubing different? How does it shrink when heated instead of expanding?
gpt
Heat shrink tubing is actually the size it is after it shrinks. The way they are manufactured is they are heated, and then expanded and allowed to cool in the expanded state. But its stretched.... it really really wants to go back to its normal size, and as soon as it's heated enough, it does.