"The Riddle of Central Park"He was

I had biked to the park
On a clear April day
To see what new riddle
The old man had to say
He was tall, tan, and charming
A real go getter 
And when it came to riddles
There was none the better.
He spun riddles about passion
Love, imagination, and fun
And he stayed hours after, past when down went the sun.
He loved riddles so much 
He probably made them up in his sleep.
Remembering names of philosophers instead of counting sheep.  But today's riddle would be a doozy; he had said so himself 
Why it might even upend the most brilliant of selves.
"What can NEVER be changed?"
NEVER be changed? 
How could he be so vague?
Why, this would stump the greatest scholars, no matter how much they were paid!
Let's see.  What can never be changed?  Someone so set in their ways?  No!  That's too ridiculous!
And still much too vague.
I know the answer; I just swear I do
For something inside is saying it's true.
Ah!  I know now!  It's cheesy, but true. 
The something that can never be changed is...a clean diaper.