#### Answer

See the explanations below.

#### Work Step by Step

Consider,
Joke 1: If you divide $6$, $973$ by $0$, you die.
The division of $6$ by $0$ is such that it gives an undefined value.
\[\frac{6}{0}=\infty \]
The division of $973$ by $0$ is such that it gives an undefined value.
\[\frac{973}{0}=\infty \]
Joke 1 describes that, no matter how many times the number is divided by $0$, the result will always be undefined or not known.
Joke 2: Once, this guy tried to find the square root of $-9$, and his eyeballs turned black.
The square root of $-9$ is $\sqrt{-9}$ and its value is not a real number but an imaginary number.
The imaginary value of $\sqrt{-9}$ is,
$\begin{align}
& \sqrt{-9}=\sqrt{\left( -1 \right)\left( 9 \right)} \\
& =\sqrt{-1}\cdot \sqrt{9} \\
& =i\sqrt{9}
\end{align}$
Where, $i$ is an imaginary unit.
Joke 2 describes that the value of a square root of a negative number $-9$ is not real but an imaginary number.
Joke 3: This girl my brother knows found out exactly what $\text{ }\!\!\pi\!\!\text{ }$ equals but she went nuts.
The number $\text{ }\!\!\pi\!\!\text{ }$ is a Greek letter and is a mathematical constant. Its value is approximately equal to $3.14159$, however it has an infinite number of digits, so finding out its "true" value would be impossible.