Why does a smell reach you even if you’re across the room? Diffusion is the invisible ‘spreading-out’ of tiny particles, and it happens all around you.

Same stuff—different behavior. In a solid, particles stay locked in place; in a liquid, they slide; in a gas, they spread out and roam.

Water can act like it has a thin stretchy skin. That’s surface tension—molecules at the surface pull together more tightly than you’d expect.

Why does hot air rise? Warm fluid becomes less dense and floats upward while cooler fluid sinks—creating a circulating conveyor belt called convection.

Sunlight gets in easily, but some of Earth’s heat has a harder time getting back out. That ‘trapping’ effect is why greenhouse gases matter.

Heat is energy moving from one place to another. Temperature is how fast particles are jiggling on average. Same word in conversation—different idea in science.

Mass is how much ‘stuff’ you have. Weight is the gravitational pull on that mass. Same object—same mass everywhere—but weight changes on the Moon.

A mirror makes light bounce back (reflection). Water makes light bend as it enters (refraction). That’s why a straw looks ‘broken’ at the waterline.

Frequency changes pitch (high vs low). Amplitude changes loudness (quiet vs loud). Same wave—two knobs you can turn.

Why do some things float and others sink? It’s a contest between weight pulling down and buoyancy pushing up—and density decides who wins.

Sprinkle salt on ice and it starts melting—even below 0°C. Salt makes it harder for water to freeze into an orderly crystal.

Seasons aren’t caused by Earth being closer to the Sun. They happen because Earth’s axis is tilted—changing how directly sunlight hits each hemisphere through the year.

Lightning and thunder happen at the same time, but light travels much faster than sound—so your eyes get the message first.

When you push something, it pushes back equally. That’s why rockets work: pushing exhaust backward pushes the rocket forward.

A pendulum trades height for speed and back again. Energy changes form, but the total stays (almost) the same.

Same force, smaller area → bigger pressure. That’s why sharp blades cut and snowshoes don’t sink.

Move a magnet near a coil and you can generate electricity. The key is change—a changing magnetic field creates an electric push.

A quick smack and a longer shove can cause the same change in motion. Momentum is what’s moving; impulse is how hard (and how long) you push to change it.

Voltage is the push. Current is the flow. Resistance decides how much flow you get from a given push.

Series is one path. Parallel is many paths. That one wiring choice changes brightness, current split, and what happens when something breaks.

DC is one direction. AC reverses direction back and forth. That’s why transformers work and why the grid uses AC.

Tiny pushes can build huge motion when the timing matches the system’s natural rhythm. That’s resonance.

Vibrations fade because energy leaks away each cycle. That energy loss is damping.

Two waves can lock into a pattern: some points don’t move (nodes), others move most (antinodes). That fixed pattern is a standing wave.

A moving source makes wavefronts bunch up in front and spread out behind—so the observed frequency shifts. That’s the Doppler effect.

Rotate one polarizing filter in front of another and light fades out. Polarization is about allowed vibration directions.

Soap bubble colors come from interference: reflections from a thin film add or cancel depending on thickness and wavelength.

A gas spreads out because there are far more ways to be spread out than piled in one corner. Entropy tracks how many microscopic arrangements are possible.

Diffusion spreads by random motion. Advection carries material with a bulk flow. Same dye, different transport mechanism.

Laminar flow is smooth and layered. Turbulent flow swirls and mixes. Increase speed and the flow can flip from calm to chaotic.

Reynolds number predicts whether flow stays smooth or turns turbulent. It’s the balance of inertia trying to stir things up vs viscosity trying to smooth them out.

Near a surface, fluid ‘sticks’ and slows down. That thin slow region is the boundary layer—and it controls drag and separation.

Tilt a wing slightly and the airflow bends downward. That momentum change creates an upward force—lift—along with pressure differences around the airfoil.

A controller constantly compares ‘where you are’ to ‘where you want to be’ and nudges the system back. That loop is feedback control.

A signal is the pattern you care about. Noise is the random junk on top. SNR tells you whether the pattern is still readable.

A complex wave can be built by adding simple sine waves. The Fourier transform tells you which frequencies (and how much) are inside a signal.

Sample a fast pattern too slowly and it can look like a different slower pattern. That illusion is aliasing.

A capacitor stores energy in an electric field. It can soak up charge, then release it—like a spring storing and returning energy.

Take random steps and you don’t go nowhere—you drift outward slowly. Typical distance from the start grows like the square root of time.

A positive test doesn’t always mean you ‘probably have it.’ Bayes updates probability using base rates plus test accuracy.

To minimize a loss, move in the direction of steepest decrease—like a ball rolling downhill. Step size controls speed vs overshoot.

Average items in a system equals arrival rate times average time inside. That’s Little’s Law—and it explains why latency explodes near capacity.

A hash maps many possible inputs into a limited set of buckets. Eventually two different inputs land in the same bucket—that’s a collision.

Batch is like doing laundry once a week; streaming is like washing each dish as soon as you use it. Same goal—different timing.

ETL is chopping ingredients before the pot; ELT is dumping everything into the pot first, then cooking and seasoning inside.

A data lake is everything poured in as-is. A data warehouse is labeled shelves where everything is organized for fast picking.

Partitioning is like a library organized by aisle—if you need ‘History’, you don’t walk every aisle. You jump straight to the right section.

Idempotency is like pressing an elevator button that lights up once—pressing it again doesn’t call two elevators.

Dedup is like stamping identical documents with the same fingerprint—duplicates collapse into one.

Training on your test data is like studying with an answer key during the exam—you’ll feel great, but you didn’t learn.

Overfitting is like tracing one exact route to school—one detour and you’re lost. The model memorizes quirks instead of learning the pattern.

Embeddings are like a map of meaning: ‘Paris’ is closer to ‘France’ than to ‘Banana’. Similar things cluster nearby.

RAG is like writing with an open book: first you fetch the right page, then you answer from it.