Replacing first occurence in every line

Question

Supposing I have a document formatted like this:

word1 word2 word3
word4 word5 word6
word7 word8

How do I use sed to replace the f

Answer 1

Simple:

sed -i 's/ /, /' your_file

This looks for the first occurrence of a space, replaces it with a comma and space, then moves to the next line.