Open any book on dinosaurs, and it will almost certainly mention their size. Some might even suggest “a T-rex weighed as much as a bus!” Dinosaurs are well known for their enormous sizes, and indeed they include the largest animals ever to walk the earth. But how do we know how heavy they were? Dinosaurs have been extinct for a long time, and only their bones remain. So how can we possibly weigh them? This activity combines palaeontology, mathematics, physics and biology, using an age-old technique to estimate just how large these amazing animals once were!