ELI5: Why do babies constantly flail their arms and legs?

Babies constantly flail their arms and legs because their brains are still learning how to control their muscles and navigate their brand-new bodies.

Imagine you just sat down in the cockpit of a giant robot, but nobody gave you the instruction manual. You would probably push every button, pull every lever, and wiggle every joystick just to see what happens. That is exactly what a baby is doing with their limbs.

Here is why all that wiggling happens during everyday life:

• Testing the controls: Inside the baby's body, the nervous system is like a bundle of telephone wires connecting the brain to the muscles. At first, the brain sends very loud, messy signals down the wires. When the baby thinks "move," the message goes everywhere at once, making their whole body kick and wave like a little wind-up toy.
• Building strength: Kicking and punching the air is a baby's version of going to the gym. Every tiny kick builds muscle tone so they can eventually roll over, sit up, crawl, and walk.
• Surprise reflexes: Newborns have automatic movements they cannot control yet. One common example is the startle reflex. If a baby hears a loud door slam or feels like they are tipping backward, their arms automatically shoot out wide like they are trying to catch a falling star, and then pull back in.
• Expressing big feelings: Babies cannot talk, so they use their whole bodies to speak. When a baby sees a bottle or their parent's smiling face, they might wave their arms wildly because they are super excited. When they are tired or gassy, they might kick hard to say, "I need help!"