Probably my most celebrated one was when I was about 10, and managed to lay hands on a bottle of potassium nitrate (my parents were pretty good about letting me spend my own cash on what I wanted), because Mom didn’t realize what it could be used for.
Combine that (70%) with flowers of sulfur (20%, on hand for tick & chigger repellent at scout campouts), and a ground up charcoal briquette filched from a friend’s dad’s barbecue pit supplies, and you have a recipe for very crude gunpowder.
I ground it with a flat brick on a paving stone until it was very powdery, heaped it up, and ignited it with a match taped to a long stick. It worked! It burned with a fairly surprising whooshing/hissing noise, created a LOT of stinky white smoke, and left the weirdest, gunkiest orange-hot, bubbling residue on the paving stone.
I did this many times when mom and dad would leave me at home- basically I’d meticulously clean up, and put the paving stone back upside down, so that the scorch marks and residue were down.
Eventually, they came home early, and Mom asked “What are you doing?” My response was truthful; being caught red-handed, I figured lying would get me in more trouble.
“Making gunpowder. It works too.”
Mom shut the door, went inside, and a minute later, out comes Dad. I’m terrified at this point, until he says “Gunpowder? And it works?”
“Uh, yeah- let me show you.” So I lit off my latest batch, and he watched with a sort of startled wonder on his face.
I’ll never forget the next words out of his mouth. “Hey- cool! Let’s make a bomb!”
So we ground up a pretty large quantity of this crude black powder, made a fairly tight tube out of notebook paper and scotch tape, and stuffed both ends with match heads (we didn’t know how to make fuses at the time), and sealed it up tight. We then built a little fire in the middle of the backyard, and tossed the “bomb” in. After about 3 minutes, apparently one end burned through first and the match-heads lit, and we had a rocket that shot about 40 feet across the yard, and floated gracefully in flames to earth. We were just as happy as if it had blown up.