How to make it so function only works when I call it?

For example, I only want this script to work when I call the function:

function timer()
while true do
for i = 25,0,-1 do
script.Parent.Num.SurfaceGui.status.Text = tostring(i)
wait(1)
end
end
end

But, the script works even without me calling the function. How do I solve this?