Prior to the Kel-Lite, flashlights were relatively fragile and undependable, usually made either of molded plastic or stamped sheet metal. Dropping one would usually break the bulb, and dents would often lead to frequent replacement. Keller designed the Kel-Lite to be robust. Salesmen trained by Keller would demonstrate this for public safety customers by pounding large nails into a block of wood, while the light was operating. Other demonstrations included throwing the light as hard as possible across a concrete parking lot, or turning the light on and putting it into an aquarium and driving over it with a truck.