Director Tim Burton used real squirrels for the movie Charlie and the Chocolate Factory because he wanted as much reality as possible. How did they do it? "Training is based on food rewards, so the squirrels got nuts when they did what was required. "You shape their behaviour so if you're running them from A to B - which could be 100m - you put catching boxes inches away so they run into the box and get a reward. Then you put the box further and further away." Some worked harder than others, while some filled up on nuts very quickly and then lost interest, he says."