Hi all

What is the logic behind the number of times a function is
called? A simple example to illustrate:

Function add(a, b)
MsgBox ("add")
add = a + b
End Function

In the worksheet I have:
A1: 1
A2: 2
A3: =add(A1;A2)

If I change the value in cell A1, the function runs 3
times. Why? Is it possible to force a certain function to
run only once?