Why does life on Earth use almost exclusively 'left-handed' amino acids and 'right-handed' sugars, when the same basic chemistry could in principle work equally well with the mirror-image molecules, and what process originally broke this molecular symmetry?
Physics
Post a problemHow do migratory animals like birds, sea turtles, and some fish sense Earth's magnetic field well enough to navigate across continents or oceans, and does this rely on a genuinely quantum-mechanical process inside a light-sensitive protein in their eyes?
A person wearing perfume enters a lecture hall; roughly how long does it take before someone across the room can smell it, and does ordinary molecular diffusion explain the timescale?
Liquid nitrogen is around -196 C, yet a small splash on bare skin often causes no injury at all: what protects the skin, and when does that protection fail?
Why does dragging a moistened finger around the rim of a wine glass produce a sustained musical tone, and why does adding water to the glass change the pitch?
Why does a droplet of water dropped onto a very hot skillet (well above boiling point) form a levitating bead that skitters around for a surprisingly long time instead of flattening and evaporating immediately?
Why does a spilled drop of coffee (or red wine) dry into a dark ring around the outside with a pale center, instead of leaving an evenly colored stain?
Why does a plastic shower curtain often billow inward toward the water spray and stick to your legs while you shower, instead of staying put or blowing outward?
Contrary to the classic cartoon teardrop symbol, what shape does a falling raindrop actually have, and why isn't it pointed at the top?
Why does burning firewood make crackling, popping, and hissing sounds, and why is the noise so much louder and more frequent with some pieces of wood than others?
On a clear day, why does the sky directly overhead look a deep, saturated blue while the sky near the horizon looks pale, washed-out blue or nearly white?
Achilles, though ten times faster than a tortoise with a head start, seems logically unable to ever catch it, since by the time he reaches where the tortoise was, it has always moved a little further ahead.
When several bodies move relative to one another (e.g., a person walking inside a moving train, or a boat crossing a flowing river), why does naively adding or subtracting the given speeds as plain numbers often give an obviously wrong resulting speed or direction?
If you travel the first half of a trip's distance at 60 km/h and the second half at 30 km/h, is your average speed for the whole trip the simple arithmetic mean, 45 km/h?
A subway rider who travels at random times notices that trains heading in his preferred direction seem to arrive less often than trains going the other way, even though the timetable runs both directions with exactly equal frequency — how can this be?
Inside an accelerating vehicle, a ball resting on a frictionless floor suddenly starts rolling backward even though nothing visibly pushes it — apparently violating Newton's first law that a body at rest stays at rest unless acted on by a force.
Why does the water pressure, and hence the total force, on the bottom of a container depend only on the height of the water column and the base area — not on the container's shape or the total weight of water inside it — so that a narrow vase can press on its base just as hard as, or harder than, a…
When two lumps of soft clay collide head-on and stick together, coming to a complete stop with no visible flash, sound, or motion afterward, where did all the kinetic energy go, if energy is supposed to be conserved?
Why does the free end of a chain falling off a table, or piling onto the floor, accelerate downward faster than gravity alone (g) would predict?
Why do raindrops in a storm follow a characteristic size distribution -- many small drops, few large ones -- as described empirically since the 1940s?