Running a marathon is hard. At least it is for a human. But what if we could swap our legs with an animal - would that make it any easier? First, we try on a pair of kangaroo legs. Rather than jump from one leg to the other, kangaroos keep their legs together and store the energy from each bounce in special ligaments that act like springs. They recycle the energy so that the kangaroo can travel long distances with very little effort. This efficient style of locomotion helps them get around the Australian bush, where it can be a long way between places with food and water. However, they get tired after around 30km and so a 42km marathon would be too much for them. So what about the longer legs of a T. rex? They have a stride length of around 4 metres compared to 1 metre for a human. And they can travel up to 50km. However, scientists believe that T. rex was too heavy to be able to run - if they tried they'd probably break their legs. So maybe we're stuck with our human legs. But don't worry - when it comes to running marathons, humans are the best animals around because we have something that other animals don't - sweat! Sweat helps keep us cool and means we can keep going for long distances without overheating. Go humans!