If you buy potatoes that have an eye, cut that part and you can stick them in a pot or in the ground and you can grow your own potatoes. Don't chuck the bottom bit of celery or bok choy - you can re-grow them. Usually, I will put the bottom bit of celery or bok choy in the saucer filled with water. It will re-grow a new leaf after some time. After that, I will plant them in a pot or in the ground. You can also try it with the top of a carrot.
Put bok choy bottom bit in saucers filled with water