Opposable describes a thumb or toe that can move and press against the other fingers or toes on the same hand or foot. Your thumb is opposable because you can touch it to each of your fingertips, creating a firm grip.

This might sound like a small thing, but opposable thumbs changed everything for humans and our primate relatives. Try picking up a pencil without using your thumb, or imagine opening a jar, tying your shoes, or playing a musical instrument. Nearly impossible, right? That's because opposable thumbs give us what scientists call a precision grip, letting us hold and manipulate small objects with control.

Most animals don't have opposable thumbs. A dog's paw, for instance, has toes that all point in roughly the same direction. Some primates like chimpanzees and gorillas have opposable thumbs too, which helps them peel fruit, groom each other, and use simple tools. Koalas also have opposable digits on both their front and back paws, perfect for gripping eucalyptus branches as they climb.

Having opposable thumbs is one reason humans could develop tools, create art, write, and build civilizations. The next time you grip a baseball or button your shirt, you can thank your opposable thumbs for making it possible.