When I was young, I liked to fake sick so I could stay home by myself all day. I needed to figure something better than a fake hoarse throat. In the bathroom one day, I spotted two large hypo type syringes, no needles. I think they were for mixing something with the pool.
I got the idea to fill them with very warm, not hot, water and keep it around in the early AM. I would play sick and let dad take my temp in my mouth. It would be fine. I'd tell him it wasnt' accurate or something like that and that , as much as I hated to (hah), we might have to take it, the OTHER way. He'd done it before so it wasn't much of a stretch.
This time, while he was fetching the 'other' thermometer and Vaseline, I would quickly inject that syringe of warm water at just the right time so when the thermometer were removed from my pseudo-reluctant bum it would show a fever.
It took a bit of experimenting, the second time I did it, I got it a little warm and the thermometer read about 105 or 6. I just about got taken to the ER but when it was taken about 30 min later, it was down to normal.
I only did this about 3 or 4 times, it worked twice.