How to sort millions of rows of data in a file with less/meagre memory

Question

(From here)

I attended an interview last week and this question was asked:

How do you sort a billion rows of data in a file with only 640KB of memory in

Answer 1

If speed is not a requirement, you could bubble sort rows in place in the file. This only requires looking at two rows of data at a time, with no external information or storage required.