Two bones, side by side
Your forearm is the part of your arm between your elbow and your wrist. Inside it are two long bones, lying side by side.
The radius is on the thumb side. The ulna is on the little finger side. The ulna is a bit longer.
Try it: hold out your arm, palm up. Feel along the little finger edge of your forearm, from your wrist to your elbow. That hard edge is your ulna, right under the skin.
Turn your palm up and down
Hold your arm out with your palm facing up, like you're holding a bowl of soup. Now turn it so your palm faces down. What just happened inside?
Your radius rolled over your ulna! When your palm faces up, the two bones lie side by side. When your palm faces down, the radius crosses over the ulna, like an X.
This twist lets you turn a doorknob, use a screwdriver, pour a drink and flip a pancake.
Try it: hold your forearm with your other hand while you twist. Can you feel the bone on the thumb side moving?
The point of your elbow
Bend your arm and touch the pointy tip of your elbow. That point is the top end of your ulna.
The ulna wraps around the bottom of your upper arm bone like a hook. That makes a hinge, so your elbow bends back and forth.
The top of the radius is round and flat, like a coin. It spins in place next to the ulna when you turn your hand.
The wrist end
At your wrist, the radius is the wide one. It takes most of the push when you lean on your hand or catch yourself.
That's why, when kids fall onto a hand, the end of the radius near the wrist is one of the most common places to break a bone. It usually heals well in a cast.
Feel the bony bump on the little finger side of your wrist. That's the bottom end of your ulna.