My son is very polite. He knows when to say thank you, and he says it often. However, he doesn't say it correctly. I've since learned that it's normal for kids to pronounce the "th-" sound as an "f-" sound until they can master that blend, but I hadn't realized how pronounced it could be.
For the longest time, whenever Drew thanks us, it sounds like he's cussing at us.
"You did such a great job, Andrew!"
"F*&k you, Dada!"
"Um, let's try more of a T sound... TANK you, Dada!"
"Ta- F*&k you, Dada!"