Hey I was watching a TV show called QI on BBC and there was a question which went along the lines of: If you weight a lorry full of chickens. Then all the chickens jump off the ground at the same instant, in that lorry, the lorry doesn't loose any weight. Then it went on to say, if the lorry was open top, and the chickens jumped, it will only lose weight if the chickens jumped higher than the lorry's height - but any lower and the lorry's weight doesn't go down. But to me I don't see how this can be possible. If all the chickens are airborne how are they adding to the weight of the lorry? I tried to find the clip on youtube, no such luck but its on BBC Iplayer who those who have access to watch the episode: http://www.bbc.co.uk/iplayer/episode/b00vs4h4/QI_Series_H_Hypothetical/ Hypothetical is the episode in which they mention it. For me I don't see how it can't lose weight, can some one explain it in a more scientific way ?