Your thumb is the fifth digit on your hand, the one that doesn't quite match your fingers. Thumbs help us grip objects with care and precision — it would be difficult to hold chopsticks without a thumb.
Humans are more dextrous than our closest primate relatives because of our nimble thumbs. They're opposable, so we can move them forward and touch them to our fingers. Some monkeys share this trait, and they're the best able to use primitive tools. Your thumbs make it possible for you to brush your teeth, write with a pen, and snap. You can also thumb through the pages of a book — in other words, turn each page with your thumb.