A black hole is matter packed so tightly that not even light escapes. We know they exist from their effects on nearby stars and gas, from X-rays, gravitational waves and one direct image.
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The Moon's gravity pulls unevenly across Earth, stretching the oceans into two bulges. Earth rotates through them, giving most coasts two high tides a day.
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Earth's orbit takes about 365.24 days, not a whole number. An extra day roughly every four years keeps the calendar in step with the seasons.
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Fission splits a heavy nucleus into lighter ones that are more tightly bound per particle, so some mass becomes energy, mostly as motion of the fragments, which turns into heat.
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Ole Roemer made the first measurement in the 1670s from timing eclipses of Jupiter's moon Io. Armand Fizeau made the first Earth-based measurement in 1849 using a toothed wheel.
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Entropy counts possible arrangements of a system and is tied to missing information. Its tendency to rise is the usual basis for time's direction, though the details remain actively studied.
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Mendeleev ordered elements by atomic weight and chemical similarity, left gaps where the pattern broke, and used neighbouring elements to predict the missing ones' properties.
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Ice floats because hydrogen bonds lock water molecules into an open crystal lattice that is less dense than liquid water. This lets lakes freeze from the top down, protecting aquatic life.
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Honey has very little available water, is acidic, and contains some hydrogen peroxide, so microbes cannot grow in it. It stays stable as long as it is sealed and kept dry.
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Ice cores preserve layers of snow, trapped air bubbles, dust and water isotopes. Together these record past temperature, greenhouse gases and atmospheric conditions, in some cases back 800,000 years.
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Plate tectonics says Earth's rigid outer shell is broken into moving plates. It was assembled in the 1950s and 1960s from seafloor mapping, magnetic stripes, drilling and earthquake data.
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Sunlight warms Earth's surface, which radiates infrared heat. Greenhouse gases absorb much of it and re-radiate it, slowing heat loss to space and keeping the planet warmer.
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Air molecules scatter short-wavelength blue light far more than red light. Overhead, that makes the sky blue; at sunset the long light path removes blue and leaves red.
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Rainbows form when sunlight is refracted, reflected inside, and refracted again by water droplets, splitting it into colours seen only from the right position.
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A mirror reverses only front and back, along the axis perpendicular to its surface. The left-right flip comes from how we imagine turning ourselves around, and specialists still frame the puzzle in different ways.
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Radiocarbon dating measures how much carbon-14 remains in once-living material. The decay is steady, so the remainder indicates time since death, after calibration.
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Most specialists attribute the extinction of the non-bird dinosaurs 66 million years ago to the Chicxulub asteroid impact, backed by iridium, a crater, and global ejecta layers.
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Around 240 BC, Eratosthenes compared the Sun's shadow angle at Alexandria with the overhead Sun at Syene, then scaled up the distance between them to get Earth's circumference.
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Ants, birds and traders each follow local rules and respond to neighbors or feedback. Group patterns emerge with no leader, a process called self-organization.
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Feedback loops either stabilize or amplify change. When amplifying loops overpower stabilizing ones past a threshold, a system can shift abruptly into a new, often hard-to-reverse state.
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The butterfly effect is sensitive dependence on initial conditions: tiny differences in a deterministic system can grow into large ones. Lorenz found it in 1961 when rounding a number changed his simulated weather.
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No. Lightning often strikes the same place repeatedly, especially tall, pointy, isolated objects, and a single flash can itself hit the same spot several times.
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No. At room temperature glass behaves as a stable solid, and uneven old window panes come from how they were made, not from flow.
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No. The wall is too narrow and too similar in color to its surroundings to be seen by the unaided eye from orbit, and it cannot be seen from the Moon.
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On 30 June 1908 an object, most likely a small asteroid, exploded in the air over remote Siberia, flattening a huge area of forest without leaving a crater.
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The Wow! signal was a strong, narrowband radio burst recorded in Ohio on August 15, 1977. It has never repeated, and its origin remains unexplained.
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