What is an ashtray?

This is one question thrown at me this week by my 12 year old son. Since we don't smoke there have never been ashtray's in the house, there wouldn't have been any at his school, there aren't any at public buildings he visits, there aren't any references in books he reads and rarely in films he watches. It probably is a question that I should have expected, but having the question asked about an object often taken for granted as being part of the furniture shows how far society has gone in removing the smoking habit from everyday life. Carry on I say.