Why do scientist use a joule instead of calories?

Scientists use both joules and calories. They are two different things. That cake that you just ate is a whole lot of calories. It is making you fat as a pig. You had better do something about it. You look in the mirror and decide it is now or never. You put on your jogging shoes and start moving. You look at your watch and take off. As you move and use calories and time, you are using joules.