Estimate the size of a needle's tip.
Physics
Post a problemIf a lawn sprinkler is submerged and made to suck water in instead of spray it out, which way does it spin?
What actually makes ice slippery enough to skate on: pressure melting, friction heating, or an intrinsically liquid-like surface layer?
Under some conditions can initially hotter water freeze faster than initially cooler water placed in the same freezer?
What physical process converts the collapse of a tiny gas bubble driven by intense sound waves in a liquid into a picosecond flash of visible/UV light, with the bubble's interior briefly reaching temperatures rivaling the Sun's surface or hotter?
Why does liquid poured slowly from a spout often dribble down the outside of the spout instead of falling cleanly into the cup?
What inside a whistling kettle actually produces the sound as the water boils?
Why does a drying drop of coffee (or any particle-laden liquid) leave a dense ring stain at its edge rather than a uniform spot?
Why can a thin falling stream of shear-thinning liquid (like shampoo) spontaneously leap upward off the pool it lands in?
Why do small floating objects like breakfast cereal clump together at the edge of a bowl of milk?
Why do tea leaves stirred into a cup collect at the center of the bottom rather than being flung outward to the rim?
Why do the bubbles in a freshly poured glass of Guinness appear to sink downward near the glass wall instead of rising?
Why does a thin sheet of water spreading out from a falling stream on a flat surface suddenly jump to a thicker, turbulent ring at a fixed radius?
Why does a liquid drop hitting a dry surface sometimes splash into a crown of droplets and sometimes spread out smoothly instead?
Why does a thin stream of honey or syrup poured onto a surface coil up like a rope instead of falling straight down?
Why does a chain of beads poured out of a beaker rise up in an arc above the rim before falling, as if pulled up by an invisible force?
How do baby water striders generate enough thrust to skate across water when their legs move too slowly to create the surface waves that classical theory said were needed?
Why does a shower curtain billow inward and cling to a bather when the shower is running?
Why does coffee in a mug slosh out and spill specifically when carried while walking, often after a fairly predictable number of steps?
Why does an air bubble rising through water often not go straight up but instead zigzag or spiral once it grows past a certain size?