Babies generally cannot sit up by themselves until they are 4-8 months old.

As they learn to keep their balance, they fall over.

A lot.

They don't like it.

But their subsequent cry depends on the type of bonk. Sometimes it's just scary (They pause very briefly with a startled look on their face, then close their eyes and cry intermittently), other times it's frustrating (They usually start crying on the way down, kind of like "Mayday! Mayday! Gyroscope malfunction! I'm losing control - bonk! - AAAAAAAAH!").

Then there are those times it really, really hurts. You know when someone hits you in the stomach and you just can't get any air in you to scream, but your mouth is wide open ready to go any time your lungs are ready? That's it.

They close their little eyes really tight, their mouth is wide open, body so tense it almost shivers, arms and legs outstretched, fingers and toes clenched, little tears starting streams down their cheeks... and they are like that for what seem like an eternity, until they (finally!) get a big breath and start wailing!

And there's nothing you can do about it. All you can hope is that by holding them safely and securely, using a comforting voice, and waiting they will calm down enough to eat, be distracted, or sleep. It is rather upsetting for the parents, and everyone else stands around just feeling the pain, awkwardly wondering if there's something they should be doing to help.

But it's nothing like the "I'm getting my first shot" cry.