Since animals emerged on land more than 300 million years ago, they've turned to plants as a source of food. But despite deploying their best modes of attack, surprisingly, animals only manage to eat around a fifth of the vegetation that’s available to them. So why don't they eat more? In her first lecture of the series, Sue Hartley explains why animals have a harder time than we think when it comes to eating plants, whilst Adam squirm as he eats the world’s hottest chilli.