Henri Poincaré, the famous French mathematician, once grew suspicious and started to weigh the bread loaves he was getting at the bakery. Indeed, he discovered that the average weight was 900 grammes (rather than 1 kg, as required). Accordingly, he reported the matter to the prefecture.
Some weeks later, friends asked him if the baker had stopped cheating, and if he was satisfied. "Oh no, he's still cheating," he replied, "but I'm now getting full weight loaves, so I'm satisfied."
Mystified, they asked him how he could know this, if he wasn't being cheated any longer. His reply? "I continued to weigh my bread; its distribution is now that of a normal distribution around 900 grammes, but truncated below 1 kg -- I only see the part of the distribution above 1kg."
In non-mathspeak, this means the baker was still cheating on the loaves, but giving Poincaré only those deviants with higher weights.