EWOT

I was doing homework in my dorm, and my friend knocked my door and asked me to do the laundry with him if I need to, and I happened to have some clothes to wash, then I made a trade with him. I helped him to figure out a math question, and he went to do both his and mine laundry. This way, I just need to spend a little more time then I would, but less than If I went to do the laundry. He needed to do the laundry anyway, and he might cannot work out the problem by using the time that he did for my laundry, I explained the problem to him when he came back. So at end, we both spent less time on doing both things, we benefited from each other. This is how trade makes us rich.