How many bones are in the human body?
Babies start with about 300 bones; many fuse, leaving adults with 206. Over half of them are in your hands and feet.
Just know.
12 facts — science, space, the body, the world. No fluff.What is the largest human organ?
The skin — around 2 square metres and 16% of your body weight. It regulates temperature, blocks microbes and makes vitamin D.
How strong is human hair?
A single strand can hold about 100 grams; a full head of hair could theoretically lift two elephants. Copper wire of the same thickness is weaker.
Why do we sleep?
Sleep clears metabolic waste from the brain, consolidates memory and repairs tissue. Adults need 7–9 hours; chronic shortage measurably impairs judgement like alcohol does.
How many bacteria live in your gut?
Tens of trillions — roughly as many bacterial cells as human cells in your body. They digest fibre, train the immune system and make vitamins.
How fast do nerve signals travel?
The fastest nerve fibres conduct at up to about 430 km/h — the reflex that yanks your hand off a hot pan outruns conscious thought.
Why do we yawn?
The leading theory: yawning cools the brain. It's also contagious — seeing, hearing or even reading about yawning triggers it in most people.
Does hair grow after death?
No — it only looks that way. Dehydrating skin retracts, exposing more of the stubble and whiskers that were there all along.
What makes blood types different?
Sugars and proteins on red blood cell surfaces. Type O-negative can be given to almost anyone in an emergency; AB-positive can receive from everyone.
How much blood does a person have?
About 5 litres in an average adult — roughly 7–8% of body weight. Donating one pint is safely replaced within weeks.
Why do fingers wrinkle in water?
It's not passive swelling — your nervous system actively constricts blood vessels, creating grip treads like tyre treads for wet surfaces.
What is herd immunity?
When enough people are immune, a pathogen can't find new hosts and outbreaks die out even among the unprotected. The threshold varies — measles needs about 95%.