What is the historical reason for the use of the Greek letter Pi to represent the circle constant (3.14159...) in modern mathematics?
The Greek letter Pi (π) was first used to represent the ratio of a circle's circumference to its diameter by the mathematician Leonhard Euler in the 1700s.

The choice of π was arbitrary, but the Greek letter π was a natural choice because:

1. It is a simple and familiar letter from the Greek alphabet.

2. The Greek alphabet is commonly used to represent constants and variables in mathematics. Many other important constants are denoted by Greek letters, like eta (η) or omega (ω).

3. The sound of π is suggestive of the English word 'perimeter', relating the constant to a key property of circles.

So π was chosen as a symbol mainly out of convenience, but its selection has endured because it is now a very familiar convention.
That's really interesting, are there any Pi jokes that would be funny to non-mathematicians?
Here are some jokes you can try with non-mathematicians.

Why should you never start talking to pi at a party?
Because it just goes on forever.

Why is pi so lucky in love?
Because its love is infinite and non-repeating.

What did the students say about their Euclidean geometry test?
It was easy as pi.

Even more optimal then trying to figure out pi jokes for non-mathematicians is to turn the person that you are trying to tell the joke to into a mathematician.