Alexis Wajsbrot is a visual effects specialist who has worked on a number of high-profile films including Harry Potter and the Half-blood Prince, Harry Potter and the Order of Phoenix, Tim Burton's Sweeney Todd, and also on some of those visually stunning commercials you see while waiting for your film to start. His speciality is anything that behaves like a fluid: water, smoke, fire, even fur or cloth. Plus went to see him to find out more.

Digital weight loss for Shrek and his virtual 3D buddies!

In the real world, balls bounce and water splashes because of the laws of physics. In computer games, a physics engine ensures the virtual world behaves realistically. Mathematician and computer programmer Nick Gray tells us about playing God in a virtual world.