Tonight, on every coast, the sea will rise. The Moon's gravity raises the ocean a little. We call that rise the tide. Now here is a single drop of that same seawater. Inside the drop sit particles far too small to see. The particles jitter all the time. They bump into one another, over and over.
The two events could hardly be more different in size. One stretches from the Moon to a whole ocean. The other fits inside one drop. Yet one subject explains them both. The subject that explains events at every size is called physics.
You have already met the big end: stars gather into galaxies, and galaxies fill the universe.
You have already met the small end: every object around you is made of atoms, and each atom holds still smaller particles inside it.
Here is the whole span of sizes, drawn as a ladder. The biggest thing sits on the top rung. Every rung below holds something smaller.
The tide sits near the top of the ladder: the Moon's gravity raising an ocean. The jitter sits near the bottom: particle bumping particle. Physics covers the tide, the jitter, and everything in between.
The lessons ahead cover the ladder from top to bottom. The next topic teaches the first tool physics needs at any size: measurement.