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

Using awk

awk '{sub(/ /,", ")}1' file
word1, word2 word3
word4, word5 word6
word7, word8