Taught myself how to program a Texas Instruments 84 graphical calculator to solve the common mathematical equations in high school. Shamelessly copied a lot from the internet but learned quite quickly. Things like solving 2 and 3 degree polynomials, prime factorization or calculating integrals and stuff.
I quite liked how I could make life easier for myself and eventually class mates. Just rolled into software engineering afterwards I guess. No regrets thus far...