Funny but inappropriate
I had a new-to-me student who was rather distracted by his phone. Though he was a bit older, I decided to pull out Wrinkles, my Shar-pei puppy puppet to "teach" and get his attention. Most students like Wrinkles and smile more when he comes out, but this 10yo shouted "What the fuuuuuu**"!!!
I ignored it thinking i heard wrong, but then it happened a couple more times. (When I needed to pull his attention from the phone and brought out the puppet).
The third time, I told him its better to say "What the PUP" in class lol