Why tardigrades are so hardy, how their biology is so different? How immune system and medications work. Why some plastics are recyclable and others are not.
All plastics are recyclable in the sense that they can be repurposed, but thermoset polymers cannot be repurposed to be used the same way as it was originally formed. Typically they are chopped up and used as matrix materials for asphalt or other composites. In some cases they can use solvents to break down chains into smaller building blocks. However, colloquially thermoset polymers are considered non-recyclable, while thermoplastic polymers are considered recyclable.
The difference between thermoset and thermoplastic polymers has to do with irreversible chemical bonding during curing. With thermosets, you have chemical bonds between molecules preventing deformation, whereas with thermoplastics, you just have a viscous friction between molecules that varies with temperature. If you heat up a thermoplastic, that viscous friction goes away and the plastic can be remolded.