My 10-month-old daughter's head hitting the floor was not something I saw. I heard it. The room was pitch black when she slipped off the bed's edge – which to her might as well have been the edge of the world.

The flat clunk of skin-padded bone against rock was unmistakable in the room's wee-hour silence. That was a head hitting a floor. A little head. A big, hard floor.

Then the crying started.

Say all the elitist things you want, but few owners of shag carpeted bedrooms have ever found themselves online at 3:00 am Googling the term "baby fall head hit call should hospital take?"

My girl continued screaming as I inspected the wound. No bleeding, thank goodness, but… was that… a dent? Yes, there was a clear dent in her forehead, not unlike the kind that deters the purchase of a can of fruit. My daughter's dent was a crater. A real moon feature. On close inspection, the dent featured an impression of the four-way tile seam she had hit.

Apparently, a dented head is "okay." As one family physician later told me, "babies are born dent-able."

But some potential symptoms of a baby's head injury are very much not okay, though. And it's not always that easy to suss them out. For instance, you are supposed to call your doctor if your child shows "confused thinking." While my 10-month-old did not yet speak, her two-year-old sister does – and just that morning told me she was going to "put on her alligator house poop." Had she hit her head when I wasn't looking?

Under head injuries, Seattle Children's Hospital’s "Should Your Child See a Doctor?" webpage recommends you "Call Your Doctor Now" if "You think your child has a serious injury."

So, basically: if frantic Googling has brought you to this page, call your doctor now.

I pondered what to tell my wife, searching obsessively under “baby falls.” Because adult human beings are horrible creatures, YouTube boasts a robust library of baby falling videos. One stomach-churner, titled "‪Crib Escape FAIL," shows a toddler fall headfirst from a crib, down and out of frame onto the floor. The audience is invited to use its imagination – just like in Psycho.

It turns out football and hockey players have nothing on being a three-year-old. The leading cause of unintentional injury for children is falls. And for very young children, head injuries from falls cause the majority of deaths and severe injury. One third of all fall-related emergency room visits are from children.

As a parent, falls concern me far more than other threats that just cannot seem to hold my terror. (Such as, say, "bottle tooth.") There are walker falls, high-chair falls, window falls, rocking-horse falls, crib falls, jumper falls, sofa falls, stool falls, Bumbo falls, changing-table falls, bathtub falls, slide falls, stroller falls, bicycle falls, swing falls, running falls, everyday pedestrian faceplants and, of course, bed falls. It's no accident that some of the most popular nursery rhymes deal with falls. "Broke his crown" is just a poetic way to say "intracranial epidural hematoma."

While a child's brain is like a sponge, his or her skull is unyielding, more like bone. In fact, exactly like bone. When a skull smashes into something, such as a creamy, travertine Capadocia walnut-tumbled 6 x 6 floor tile, it comes to an abrupt and violent halt.

Ever curious, the brain continues on its path to see what the holdup is, slamming into the now-stopped skull. Did you know that once the brain strikes the skull – coup injury – it may bounce back and strike another side of the skull, causing a separate countrecoup injury? Well, you do now.

More trivia: Mortality after a fall more than doubles at 15 feet. For a fall from less than five feet, such as from a bed, mortality is just 0.5 percent.

My girl's screams, quickly joined by sympathetic yelps from the dog, were by far the best signal her injury was not life threatening. If the fallen are immediately quiet – or still screaming after 20 minutes or more – that’s when worrying should really be done. After weeks spent researching online and talking to doctors, what I've learned is: vomiting, lots of blood, loss of consciousness, a seizure, fall from a height of five feet or more, or swelling beyond the size of a silver dollar are all signs it's time to go see a doctor.

Also, if you're unsure, go see a doctor.

When I finally did get my daughter snoozing again, I was haunted by Internet's horror stories about head-injured babies who never woke up. After all that screaming and drama, the poor child was so worn out that all she wanted to do was sleep it off. But for every 30 minutes, through until the morning, she had to respond to a curious poke in the ribs from the very same guy who let her fall on her head in the first place.

A week later the black-and-blue bruise, after turning a putrid shade of brown in its final days, finally cleared away. Her forehead again unblemished, the only lasting injury was my own sense of guilt – and an inability to ever forget that sound.

Illustrations: Ryan Snook