Why is a digger like a giant arm?

Have you ever watched a digger working by the road? Its long arm reaches out, dips down, and scoops up dirt. Stretch out your own arm now, and feel it bend.

Your arm can reach and scoop too. So why is a digger like a giant arm?

The digger's arm is fixed to its body, the way your arm is fixed at your shoulder. The heavy body holds the arm steady while it works.

The first long part of the arm is called the boom, which means the big piece nearest the body. The boom is like the top of your arm, from shoulder to elbow. It lifts the whole arm up high.

At the end of the boom, the digger's arm bends. That bend is a joint, which means a place where two parts meet and fold. Your elbow is a joint too, and it lets your arm fold.

After that joint comes a second long part. It is like the lower part of your arm, from elbow to wrist. It swings out to reach far, and swings in to pull close. What do you think comes at the very end, just like on you?

A bucket! The bucket is like your hand, and its metal teeth are like fingers. They bite into the hard ground.

One more joint sits between that second part and the bucket. Your wrist is a joint like that. It lets the bucket curl in to scoop, and tip open to drop the dirt.

The driver sits in the little room on top and moves the handles. The handles tell the parts of the arm when to bend. So the driver bends the digger's arm, the way you bend your own.

Inside your arm, muscles pull to bend each joint. The digger has strong metal rods instead. They push and pull the boom and bucket, much harder than your muscles can.

That is why the digger is like a giant arm. Its parts are huge and strong. So it lifts heavy rocks you could never carry.

A digger's boom is like the top of your arm. Its joints bend like your elbow and wrist, and its bucket scoops like your hand.

Out by the road, the big arm reaches and bends at its joints. Its bucket scoops one more load of dirt.