ELI5

2017-06-01 21:28:44 Yes, fire did burn differently because of excess oxygen fuel. During the carboniferous period, 300-350 million years ago, oxygen levels reached as high as 35%, compared to today's ~20%. This allowed fires to start in places which we wouldn't normally expect (places of high humidity, during rain storms). Today we find fossilized charcoal in coal beds that arose from peat bogs, which grants credence to the idea that fires did burn much more easily during this period. You might ask "how did plants survive the constant danger of fire?" Adaptation thanks to random evolution. For example, plants from this period expressed deeper roots/tubers than their modern day counterparts, and the leaves of trees were higher which could help avoid catching fire from a stray brush fire.
The reason so much coal (a large part of which is oftentimes fossilized charcoal) remains from this period is due to two factors, as other posters have mentioned. First The structural material of plants, lignin, doesn't burn rapidly, (paper is often stripped free of lignin which is why it burns so easily) rather it tends to smolder (i.e. logs in a fire), so fires would leave large amounts of charcoal rather than their usual main byproducts: carbon dioxide,carbon monoxide (at lower burning efficiencies) and water. Secondly, lignin is very hard to digest, even fungus/bacteria today have a hard time digesting it. 300 million years ago, the decay rate was practically zero. In summary, pretty much all of todays coal comes from plants which, millions of years ago, died and were buried without ever going through the process of decay.
What's even cooler though is that during this time gigantic insects roamed the earth https://en.wikipedia.org/wiki/Meganeura which is believed to be related to the faster metabolic rates that high oxygen content allows. Open / Comment

2017-06-01 21:28:39 Since millions of years ago there was a much higher oxygen content, did fire behave any differently? Open / Comment

2017-06-01 21:22:12 How do seeds know which way is up? Open / Comment

2017-06-01 21:22:06 All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots - and the seed while it is underground - rely on gravity for orientation.
The mechanism is thought to be based on either the protoplasm (the living substance inside a cell) exerting a greater pressure on the cell walls at the bottom, or starch grains within the cells settling at the bottom. One or both of these cues influence the production of plant growth hormones that cause the plant to 'steer' as it grows.

Details for nerd 5 years old:
They have starchy components (amyloplasts) in some of their cells that are heavier than the cytoplasm, making them sediment at the bottom of the cell. A hormonal growth signal then emanates in the direction of the part of the cell where the amyloplasts accumulate (i.e. down).
You can see this for yourself by tipping over a fast-growing potted plant. The side of the stem facing down will grow faster, causing the shoot to gradually bend upright again. Plants use the same mechanism - uneven distribution of growth hormones in response to sensory input - when they grow toward light, etc. Open / Comment

2017-05-30 14:01:51 There is a global unified postal service body called the Universal Postal Union . The Union creates a framework and standards so that ALL countries can exchange mail freely without forming individual contracts with every other country.
When one pays for the postage for international mail, a portion of the money goes to the home country's postal service and a portion goes to the destination's postal service. The portion of money exchanged depends on how much mail (in weight) each country is exchanging. This is set up so that the destination country receives money for delivering the postage. Countries that receive more mail annually get less money per kilogram of mail.
The fee paid to the destination county is called a terminal due. Open / Comment

2017-05-30 14:01:48 When sending a letter abroad, how does the receiving country's mail service get paid for their work? Open / Comment

2017-05-30 13:34:59 This is because muscle cells (also called muscle fibers) don't divide, and cancer is most common in cells that divide a lot. Dividing cells are primed for cancer because: 1) a lot of the machinery needed to divide rapidly is already present in the cell so not a lot extra is needed to make it grow uncontrollably into a tumor/cancer 2) the more a cell divides the more chances it has to make errors. DNA replication isn't perfect, and over time these errors can accumulate into dangerous mutations (this is also why cancer is more common in the elderly) This is why we see a lot of cancer in tissues with a high "cellular turnover" rate - skin, liver, colon, etc. Cells that never divide (muscle fibers, neurons) almost never get cancer. Brain cancer is usually caused by a special type of brain cell that can divide (but not signal) call neuroglia, which forms the cancerous "glioma." Bonus biology fact - because muscle cells fibers don't divide, you have the same number your whole life. Arnold Schwarzenegger has the same number of muscle fibers now as he did as a baby, they're just more packed with the good stuff. Source - biochemistry major, medical researcher. Open / Comment

2017-05-30 13:34:58 Why is heart cancer so rare? Open / Comment

2017-05-29 22:03:23 Microwaves don't really heat food itself, they mostly heat the water inside food. But microwaves don't heat the water evenly. It gets heated in little sections, and those hot parts spread heat to all the cold ones. That takes time though. When food snaps but doesn't get hot its because the water in those hot parts started boiling (that's the sound) but it hasn't been long enough for the heat to spread out (that's why its cold). This is why packages say to let the food sit for a few minutes after cooking. Or you can run the microwave a few seconds on and a few seconds off to spread the cooking time out, which is the same exact thing the "power" setting will do. Open / Comment

2017-05-29 22:03:02 Why sometimes when you put food in the microwave you hear lots of sparking sounds, then take it out, and your food is still cold/warm? Open / Comment