Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Sally lost her kite because she wasn't careful.  She thought that there was more string on the spool, but it slipped out of her fingers when she reached the what?.
end of line
end of the spool.