How does hair know when to stop growing?

Hair does not know how long they have to grow.

But they have a finite lifetime. After that they stop growing and they fall out. Also new hair is allways growing. The equilibrium is the terminal length where you think hair is no longer growing.

Bodyhair has a much lower liftetime than head hair, that is why eyebrows are shorter than the hair on head.

If you don't cut your hair it will also reach a terminal length and it will look like its not growing anymore.

What makes a substance greasy or sticky on a molecular level?

When thinking about things being sticky or slick it's useful to think of the molecules as tiny little robots with magnetic grabber arms. Some materials have lots of arms and connect to each other well. Some materials have very few arms and don't have a strong connection.  Some materials have the wrong kind of connectors for each other and don't have a connection at all, in fact they push each other apart.

Sticky things like tape, glue, snot and honey are all made up of little parts, molecules, that have a lot of grabber arms. They are also really good at getting into all the little nooks and crannies of what it's trying to stick to.

Slick or oily things like grease, lubricants and ice don't grab on to other things as well as sticky things because they have fewer arms and don't do a good job of making contact with the other surface.

This is why pressing on things helps to make them stick better. You are forcing more of the two things to touch, getting into all the little bits of the surface. You also add energy by pressing which can cause things like forcing more grabber arms to activate.

Since the Coca Cola recipe is a secret, isn't it illegal to not list all the ingredients for a product?

They do list ingredients, as required by (U.S.) law.

CARBONATED WATER, HIGH FRUCTOSE CORN SYRUP, CARAMEL COLOR, PHOSPHORIC ACID, NATURAL FLAVORS, CAFFEINE.

But the law does not require them to spell out exactly what all those "natural flavors" are, nor the exact proportions, nor the processes they put the ingredients through - for Coca-Cola, or anything else.

The more interesting case is wine, which does not have to list ingredients in the U.S., as the law contains an exemption for wine.

Why is sore throat a symptom of many common sicknesses?

Your body is a castle. Germs are invaders. Your castle has thick walls but needs gates so you can see out of it, take in supplies and get rid of rubbish.

Gates are weak spots so get attacked more often. The throat is the most open to attack as it is moist, warm and we are constantly moving air (containing viruses/bacteria), food which we have touched with our dirty hands etc. across it.

The throat releases chemicals to make it swell up to deliver more blood (and therefore more infection fighting cells, nutrients etc.) to the site of attack. Swelling causes pain, presumably to make us go easy on parts of the body which are under attack.

Why do tablets sometimes feel like they are stuck in our throat after swallowing?

They're kind of stuck. They'll keep moving down your throat but that feeling means that the pill hit the walls of your esophagus when they were too dry so it's harder for the esophagus to move the pill down.

Think of it like trying to go down a water slide. If the slide is wet you don't get stuck and go very fast. If the slide is dry your skin creates friction and you go slow or get stuck. You need that lubrication of the water to go down fast.

If it's a common issue for you try taking pills with a bigger sip of water. Let the pill float in your mouth for a second in the water then swallow.

Why do many workplace PCs still run on old software?

A few reasons:

1.  Cost. Upgrading software is expensive.  If an upgraded licence costs $50 that may not seem like much, but doing that for 5,000 machines gets really expensive really quickly. 

2.  Compatibility. Very often, companies will have custom or specialized software that may not function well with newer software.  If the older software doesn't have patches/upgrades, or those patches/upgrades are not cost effective (see point 1) you may need to keep other software downgraded in order to preserve compatibility.

3.  Security/Reliabilty. Older software - particularly in the enterprise - can be more secure and/or stable because the bugs and security holes have been patched. Newer software may introduce newer bugs that can compromise operations.

4. Usability. Not everyone is computer literate in a company and upgrading their software may confuse them to the point where they struggle to do their job until they learn the new software.

Why does a laser pointer have a grainy pattern in the beam?

The light from a laser is coherent, all the waves of the same wavelength, color, and all in exact lockstep with one another. When the beam strikes a surface and bounces off and the wavefronts collide with one another and can reinforce and/or cancel one another.

The sparkle pattern of a laser can be compared to the choppy surface of a lake on a windy day as waves cancel and reinforce one another to the extent that no regular pattern of waves can be seen.
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