A device warns that a surface is temperature excessive. A light guide receiving light from a light source sufficiently distant from the surface to avoid becoming dangerously hot or cold is oriented so that the light guide is in the path of the light beam and transmits the light beam through the light guide to a point outside the light guide. The light source is controlled by a controller. A predetermined warning symbol typically outside the light guide is illuminated when the surface is dangerously temperature excessive because a sensor connected to the controller ascertains and transmits the surface temperature to the controller controlling the light source and an observer sees that the surface is temperature excessive. In certain embodiments, the light guide is flexible and/or its second end movable. The symbol can illuminate a border to warn against entry into an area in which the surface is located.