Why do babies
flail so much?

The full narration of the video.

Why do babies keep waving their arms and kicking their legs? Lying on the bed, little hands drumming, little feet pedaling an invisible bike — they stop for a moment, then off they go again. First, the reassuring part: nearly every baby does this.

By the second month of pregnancy, they're already moving like this in the womb. So what is all this moving for? Let's follow one kick and see where it goes. This kick wasn't planned by the brain.

The part of the brain that keeps movements on target hasn't finished growing yet. So the movements are big and wild, and they start up again on their own. The interesting part comes after the kick. The foot brushes the blanket, the knee bends and straightens, and all those feelings travel up the nerves, back to the brain.

What does the brain do with them? Researchers put soft brainwave caps on babies just two days old, and recorded while they slept. Yes — babies kick and wave even in their sleep.

Here's what they saw: the right hand moves, and right after, a burst of brainwaves fires in the small patch of the left brain that feels the right hand. Hand first, then the light. And the earlier a baby was born, the bigger the burst. Every move, the brain takes a note: this is my right hand, and here it is.

All that kicking is a baby getting to know their own body, piece by piece. It's a bit like getting a new remote with no manual: you press every button and watch what changes on the TV. The muscles are the buttons, and the feelings sent back to the brain are the changes on screen. Press enough, and you remember which button does what.

That map in the brain, one patch for each body part, is called the body map. A remote's buttons never change, but a baby's body grows every day. Arms get longer, legs get chunkier, and the map has to keep up — so the flailing goes on for months.

And they don't drill one move over and over. They try a bit of this, a bit of that, exploring every combination. Little by little, hands and feet start working together. Around four months, they can bring a hand to their mouth and swing at toys. Around six months, they can reach out and grab what they want.

Doctors use these movements to check development, too. Some hospitals film a baby moving on their back — it's called a general movements assessment. If a baby barely moves, stays stiff, or has shaking that won't stop, ask a doctor and follow their advice. Next time your baby is flailing away on the bed, don't rush to hold them still.

Count how many times their hand finds their mouth, then count again in a month. How old was your baby the first time they grabbed something right on target?