Two other innovations were introduced briefly in Hot Wheels cars in the 1980s – "Thermal Color Change" paint, and rotating 'crash panel' vehicles (""Crack-Ups""). The former was able to change color on exposure to hot or cold water, and there was an initial release of 20 different cars, available as sets of three vehicles. The latter were vehicles with a panel that, on contact, would rotate to reveal a reverse side that appeared to be heavily dented. Variations in crash-panels included front, rear and side panels, the last of whose mechanism has proven to be the most durable.