There’s a post on Reddit today titled, "This is the first thing they should teach in Trigonometry," which is where I stole this gif.
I totally agree. Apart from equal triangles, various equivalences, etc.., understanding the nature of sine and cosine is monumental.
As a teenager, I was so excited by this knowledge, I not only took one point and rotated it in a circle, I then took four points, each 90 degrees apart, connected them by lines, and rotated a square.
Then I said, what happens if you rotate that square 90 degrees into the screen and say clone it a few times? Cool. I can rotate a cube. And what happens if you take that cube and add some parallax, such that things far away move towards a vanishing point?
Pretty soon, basic trig lead me to reinvent 3D graphics, the painters algorithm for drawing polygons, and I was off. In fact, the only thing I was missing as a kid was linear algebra and the knowledge that you could actually make a living doing fun stuff like this. Had I known the latter part, I’d probably have become a game developer at an early age.