Bought some rock-hard avocados and wanted to see if there was a way to speed up the ripening process.
I recieved two suggestions on Chowhound:
1) The one I alread knew, put them in a paper bag with a cut apple or banana or
2) Bury them in flour. Which sounded strange. But Diane Kennedy was a source for this tip.
I put two of them in a paper bag with a cut apple (and a banana) and buried two in flour. After two days buried in flour, those were ready to eat. Three days after, the paper bag ones are still too hard.
I've transfered them to the flour! Thank you, susu, for that tip.