Once merged, you own it

If you merge someone's incomplete patch because it shows progress and is in the right spirit, be prepared to finish off all the other details yourself since the merge is often seen as the final station for contributors. You need to reject and ask for improvements before the code goes in to make sure the contributor stays willing to work on and polish the code.

Shoveling

Contributors sending improvements and pull requests to an Open Source project could be viewed as your friendly neighborhood people helping you out shoveling sand into a pile. The actual resulting pile of sand is yours, on your land. You get help to make the pile better and faster, sure, but the people helping you out do not consider the sand to be theirs in any way. It is yours.

A maintainer of the project is someone who actually feels some responsibility for or co-ownership of the pile, but most contributors never end up maintainers.