Sometimes I don't get the joke

My always laughing Spanish professor had a joke today. He is a 72 year old grad student.
The word "decir" in Spanish is to say, and in the past tense is "dijé, dijiste, dijó," and he was explaining how there is a vowel and stem change that occurs when you conjugate it- nothing anyone would really care about.
Then he said the word decir reminds him of a joke
"I'm just whistling dixie," he said, then he burst out in hysterics.
He asked who got it. One girl raised her hands. The other 10 people or so just kind of looked around, myself included.
Then he burst out into song and sang the whole song.
Then he asked if we knew what dixie was.
Then he asked if we knew what dixie cups were.
He asked if we got the joke again.

Here is the song...


Maybe 4-5 minutes later he has another random question.

"How many drink postum?
Positum in latin reminds me of postum.
It is like you're having a posit.
It is what we used to give kids in the morning.
AHAHAHAHAHAHAHAHAHAHA
It is made out of different cereals.
Look for it on the shelf next time you're in the supermarket.
AHAHAHAHAHAHAHAHAHAHAHA"