how io.read() will return nil ?

[lee <leeyacn@...>]

hi, list
I am a newbie to Lua and learning.

I write a test script, named test3.lua:

 for count = 1, math.huge do
     local line = io.read()
     if line == nil then break end
     io.write(string.format("%6d ", count), line, "\n")
 end

The script is very simple, but after running it, I do not know how to exit  
the script normally.
I means what I should input will make io.read() return nil.

thanks all.
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