Why were tankards originally glass-bottomed? Pewter mugs with glass bottoms are a survival of long ago days; when it was the custom to pledge friendship by drinking to the health of one another. Such healths were not always exchanged without mutual suspicion; and a glass bottom provided the drinker with the opportunity of watching for a stealthy blow. The same suspicion was apparent as the loving cup was passed round; and one guest held the cover with his dagger hand; whilst his neighbour drank to him. The superstition of passing on the stairs comes about in the same way; the fear of a stealthy blow. Illustration for one of a set of 50 "Do You Know?" cigarette cards issued by Carreras in 1939.