Ok, I feel sure there's some recursive way that would be harder to
read, and fewer lines, but this'll do:
on mouseUp
ask "How Big"
if it is not empty then answer fibber(it)
end mouseUp
function fibber n
put 0 into n1
put 1 into n2
repeat with a = 2 to n
put n1+n2 into t
put n2 into n1
put t into n2
end repeat
return t
end fibber