What is the correct way to define a function that can be plugged
into any spreadsheet cell as a formula??

I defined a function as follows:

Public Function DoMyStuff ()
importantData = Sheet1.Range("A1").Value
importantData = importantData * 10
DoMyStuff = importantData ' Return a value
End Function

I then plugged my function into cell "B1" as follows:

=DoMyStuf()

Okay, now I noticed that when anybody edits the
contents of cell "A1" that my "DoMyStuff" is not getting
called again. Do I need some code in my "DoMyStuff"
function to detect if the contents of cell "A1" changed??

Thank you!