A 200-year-old prediction about how heat crosses boundaries has now been confirmed in matter hotter than the surface of the Sun, with consequences for fusion energy. Whenever heat crosses a boundary between two different materials, it meets resistance. Right at the interface, the temperature does not fall smoothly but jumps, as if the heat were queueing at a toll booth. Heat’s hidden toll boothPhysicists call this effect interfacial thermal resistance (ITR), or sometimes Kapitza resistance, after Pyotr Kapitza, who measured it between liquid helium and solid surfaces in 1941. In HED matter, a dense sea of mobile electrons should, in principle, ferry heat straight across any interface.