How do the supermassive black holes that power quasars and sit at galaxy centers form and grow, and how do galaxies themselves assemble over cosmic time?
When a drop or stone hits a liquid surface fast enough, why does the splash form a crown of regularly-spaced spikes, and how does the crown height depend on impact speed and drop size?
Why can basilisk lizards run across open water without sinking, and could a human do the same thing at ordinary Earth gravity?
A moving bicycle with no rider steering it can recover from being pushed sideways and keep rolling upright by itself: what physical effect makes it self-stabilize?
Given a snow-covered mountain slope of a certain height and angle, roughly how fast will an avalanche be moving by the time it reaches the bottom, and how far will the debris travel?
If you send a large enough electrical current pulse through a thin wire to make it vaporize, what is the maximum magnetic field achievable near the wire before it disintegrates, and what ultimately limits the field strength reachable this way?
Neutrons, like electrons, have a wavelength that could in principle be used for microscopy: how does the achievable resolving power of a neutron microscope compare to an electron microscope, and what fundamentally limits it?
If two artificial satellites moving in opposite directions along intersecting orbits collide directly, what happens to the debris, and does this pose any longer-term hazard to other satellites?
Why does an airplane passenger or mountain hiker sometimes see concentric colored rings of light (a 'glory') surrounding the shadow of their own head, cast on clouds or fog directly opposite the sun?
Why did the newly opened Millennium footbridge in London begin swaying alarmingly from side to side as soon as a crowd walked across it, even though it had been engineered to withstand pedestrian loads?
Why do certain desert sand dunes emit a loud, sustained booming or humming sound (sometimes audible for miles) when sand cascades down their slope, while most sand dunes and beaches stay completely silent when disturbed?
When a tiny gas bubble trapped in water is driven to violently collapse by a sound wave, why does it emit a flash of light -- and why is the exact mechanism that produces that light still debated among physicists?