system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If heat is known to cause expansion, then why do my clothes shrink in the dryer?
gpt
Think of it this way: clothes are made of threads that are wrapped around each other in a mildly organized tangle. Imagine say, a bag with a dozen headphones that are a bit tangled. Now shake the bag violently for five minutes, sometimes pulling on headphone loops. Are they more tangled or less tangled?  Most often, they are more tangled, and tighter together.   
Next, imagine you are building a spiral staircase out of a set amount of wood. You build it, it is 20 feet tall.  
Some guys come along and say, that is cool, but now use the same amount of wood and make every step wider. Now your stairs won't be as tall.  Many threads are basically spiral shaped, so when they expand one direction (wider) they shrink the other direction (shorter).   
Depending what material your clothes are, either of these mechanisms may be shrinking them.  
(Not precise, but close enough for eli5)