These are some of the side projects I've worked on over the last few years, arranged approximately in descending chronological order of when I started each project.
Sometimes when I get bored, I calculate random things in Mathematica and put them on a blog, like finding the vibrational eigenmodes of a cow, or describing a particle in a fidget-spinner-shaped potential well. I've also written a practical guide to Mathematica, specifically for use in problem sets, as part of an "upperclassman workshop" that I gave in my senior year of college. The guide became quite popular and well-distributed within both my house and the more general Caltech undergraduate population.
Sometimes I amuse myself in my spare time by answering science- (especially physics-) related questions on the Ask Science subreddit. Good questions pop up a few times a week, and to date, I've answered over 150 of them. You also learn a few things in the process of explaining something to other people.
During my sophomore year of college, I spent some free time writing an (unfinished) easy-to-use grid computing framework and bought a batch of old computers on wholesale to construct a small computer cluster to test my framework on. A pair of freshmen at Caltech are continuing development on my framework using the cluster I built.
A popular plugin I wrote in 2015 that can render an arbitrary time-animated scatterplot using matplotlib and ImageMagick. I developed the framework to simplify the animation process of some complex time-of-arrival plots I was making to study electromagnetic shower dynamics while I was doing research with CERN.
Building a toy fusion reactor
During middle and high school, I built a variation on the design for a Farnsworth Fusor, which is an inertial electrostatic confinement device that can act as a weak neutron source by fusing small quantities of deuterium ions. I used my reactor to perform a series of rudimentary experiments for Intel ISEF which demonstrated a method of neutron collimation by using an additional ion gun to impart a velocity differential in the deuterium ions, skewing the momentum distribution of the neutrons produced in the fusion reactions. If you're curious, I have a (very old) Flickr album of photos of the reactor at various points in construction.