Thermal Hair is made from saran or polypropylene fiber that changes color in reaction to changes in temperature. It comes in small packages ranging from 16-25g; please refer to the item descriptions for the weights of individual colors.
Thermal Pilot Hair is similar to Thermal Hair, except it reacts and changes color much faster. Handling Thermal Pilot Hair is often enough for any spots you've touched to change color.
Both types of hair have a higher resistance to heat than kanekalon, but can still be boil permed and will melt under high, dry heat like a straightener/flat iron. Boiling water generally is not hot enough to seal them. They will not shrink when sealed, but will hold their shape once set.
If the ambient temperature of the room you're in is close to the point where the color changes, sometimes it will appear to get "stuck." For example, your hair might be purple in the bag, turn teal from the warmth of your hands when you remove it, then stay teal after you've set it down. You can get it to go back to purple by exposing it to a much colder temperature, like an ice pack.
Thermal Hair cannot "burn out" - the fiber will never stop being able to change colors.