13 Questions You Never Knew You Needed Answering, Answered

Why do you never see baby pigeons? And what causes that smell when it rains?

1. Why do old books smell like that?

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In short: several hundred volatile compounds. According to a 2009 study published in Analytical Chemistry, these volatile organic compounds are released into the air from the books' components – paper, ink, and glue – as they break down.

Lead author Matija Strlič described the smell to the American Chemical Society as "a combination of grassy notes with a tang of acids and a hint of vanilla over an underlying mustiness".

2. How do you grow seedless grapes?

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Most fruit today doesn't come from seeds, but from cuttings. A small piece of vine or branch is cut off, treated, and placed in the soil so roots and leaves form.

Some "seedless" grapes do contain seeds, they're just very small. And most grapes contain seeds at some point, they just don't form the hard outer shell they used to.

3. Why do you never see baby pigeons?

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Probably because you don't go around peering into pigeon's nests. Pigeons don't leave the nest until they are almost fully grown, and you probably couldn't tell a young pigeon from an adult one by that stage.

4. Why does rain smell so good?

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The smell of the first rain on dry earth is so distinctive it actually has a name: petrichor. The word was coined by scientists Bear and Thomas in the 1960s. The smell itself is a mixture of compounds. Geosmin, given off by bacteria in soil, adds an earthy smell (it's also found in beetroot). Other smells come from plant or animal matter that has accumulated in rocks and soil (or pavements and buildings, if you're in the city). When rain hits, these compounds are disturbed and released into the air.

During proper storms, lightning actually splits nitrogen and oxygen molecules in the atmosphere, setting off a chain of reactions that ends in ozone, which can add a distinct whiff of sparks to the air.

5. Why do you cry when you chop an onion?

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Amino acid sulfoxides are what give onions their distinct flavour. But when you cut an onion, you release enzymes that turn these molecules into sulfenic acids – and sulfenic acids sting.

They actually form two different compounds, one that causes the distinct onion smell, and a different one that makes you weepy. The formation of syn-propanethial-S-oxide – the one that causes tears – peaks at 30 seconds after cutting, and is done after five minutes.

Nerves in your eye interpret the syn-propanethial-S-oxide as a burning sensation, and your brain tells your eyes to wash away the irritant.

6. How much gold does a wedding ring lose over time?

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According to a 2008 study in Gold Bulletin, on average a ring loses 0.12mg of gold a week.

Chemist Georg Steinhauser, author of the study, writes: "The highest abrasion could be observed in the course of vacation on the beach, where obviously the friction with sand increased the abrasion." So maybe you shouldn't be wearing your wedding ring on your honeymoon.

Or look on the bright side: You're leaving tiny traces of gold wherever you go, like a sparkly magical unicorn.

7. Why does hot rubbish smell so bad?

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Most rubbish bins contain some organic material – vegetable peelings, leftover food, etc. When it's left it starts to decompose, releasing stinky compounds that are essentially telling you not to eat it. If the environment is warmer, the food will decompose quicker.

And we're not as sensitive to odours when it's cold, so any smells coming from a rubbish bin will be made worse when it's warm out.

8. Why can't penguins fly?

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Somewhere along the winding path of evolution, it seems a bird has to decide what it wants to be good at: diving, or flying. That's according to a paper published in PNAS in 2013.

The paper supports the theory that penguins can't fly because the ability to fly requires different body adaptions than the ability to dive well. "Basically, they have to reduce their wings or grow larger to improve their diving, and both would make flying impossible," Robert Ricklefs, an ornithologist at the University of Missouri–St. Louis and co-author of the paper, told Nature News.

9. Why is it so hard to sneeze with your eyes open?

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First things first: If you do manage it, your eyes will not pop out. And anyway, shutting your eyes wouldn't stop it happening if it was going to, apparently. "There is no way that keeping your eyelids closed can prevent [your eyes from popping out]," Dr. Robert Naclerio, a professor of surgery and section chief of otolaryngology at the University of Chicago Medicine, told NBC News in 2013. "It's not like the muscles are strong enough."

The real reason is more boring: It's a reflex. When your brain sends the message to sneeze, part of it tells your eyes to shut.

10. Why do humans walk forwards instead of sideways?

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Crabs can do it, so why not us? Even when we want to go sideways, humans tend to turn and walk forwards to get there. One reason for that may be that walking sideways requires as much energy as running forwards does. A study published in Biology Letters in 2013 showed that walking sideways slows us down and uses more energy because we have to repeatedly stop and start.

11. Why do some people have loads of freckles and others don't?

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Freckles contain melanin. Most freckles are caused by the same gene that causes red hair: MC1R.

The cells in the skin that make melanin are called melanocytes. MC1R makes a protein that lives on these cells and tells your body what kind of melanin to make. In people who tan, the melanocytes produce more of one kind of melanin, called eumelanin. But in other people, they make more pheomelanin – those people are pale and get freckles.

That's also why people with freckles don't really tan. Pheomelanin doesn't protect you from UV exposure, whereas eumelanin does.

12. Why does a tiny speck of dust in your eye hurt so much?

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Your cornea – the clear, outer layer of your eye that covers your iris and pupil – has lots of nerve endings. If you get some dust in your eye and then rub your eye, you can drag the dust across your cornea, making the pain worse, or even end up embedding the foreign object in the cornea. Try blinking instead of rubbing your eyes – it might be enough to dislodge it.

13. Why do pineapples taste prickly?

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Pineapples contain a mixture of enzymes called bromelain that break down proteins. In fact, it's often used as a meat tenderiser. Sadly, this means it can do weird things to your tongue, making food you eat later taste weird. Both time and cooking will break the enzyme down. That's why tinned pineapple doesn't have the same effect. If you do want fresh, try cutting it the night before and letting is rest before you eat.