I used to stick a large glass thermometer down my dick when a kid. It was from my father's lab. It was strong, big, and I was careful, but I always wondered, what would happen if it broke. It never came close to that, it really felt good sliding down my dick and then I jerked off with it removed just to be sure I didn't snap it in all of my excitement, and was I excited, really excited.
Another favorite was to stick it up my very tight asshole and take my temperature, that was hot, the Vaseline, feeling it penetrate my tight asshole (tight way back then, lol). ... but my enemas surpassed that, squatting on the toilet with a nozzle up my ass, and my mother's hook was very high on the wall, almost to the ceiling. I took a really big one back then, about 4 qts. in high school. Often I clogged the toilet with all of crap and toilet paper, fun times!!! Had to work fast when home alone before someone came back home unexpectedly!