Explanation
Boolean Functions (Expressions):
It is useful to know how many different Boolean functions can be constructed on a set of Boolean variables.
When there are no variables: There are two expressions
False = 0 and True = 1
For one variable p: Four functions can be constructed.
Recall, a function maps each input value of a variable to one and only one output value.
1. The False(p) function maps each value of p to 0 (False).
2. The identity(p) function maps each value of p to the identical value.
3. The flip(p) function maps False to True and True to False.
4. The True(p) function maps each value of p to 1 (True).
So, For one variable p, 4 = 2^1 functions can be constructed.
This information can be collected into a table

For one variable p and q, 16 Boolean functions can be constructed.
Boolean functions of n variables:

There are 22ndifferent Boolean functions on n Boolean variables.