1. A function can be called by appending '()' to its name, as in
'my_function()'.

2. A function is an expression in it's own right, and is a value. Also, when a
function is called it always returns a value. So given a function called 'f',
both 'f' and 'f()' are expressions. The former yields the value of the function
itself, as python can use functions as first-order variables, the second has the
value return by the execution of 'f'.

3. --- sum_two.py ---
a = int(raw_input("Enter a number: "))
b = int(raw_input("Enter a number: "))
print "The sum of ", a, "and", b, "is", a+b
--- end sum_two.py ---

4. --- diff_two.py ---
a = int(raw_input("Enter a number: "))
b = int(raw_input("Enter a number: "))
print "The difference between", a, "and", b, "is", a-b
--- diff_two.py ---

5. --- mul_two.py ---
a = int(raw_input("Enter a number: "))
b = int(raw_input("Enter a number: "))
print "The product of", a, "and", b, "is", a*b
--- end mul_two.py ---

6. --- repeat_str.py ---
n = int(raw_input("Enter a number: "))
t = raw_input("Enter some text to repeat: ")
print n*t
--- end repeat_str.py ---