Darning is a sewing technique for repairing holes or worn areas in fabric or knitting using needle and thread alone. It is often done by hand, but it is also possible to darn with a sewing machine. Hand darning employs the darning stitch, a simple running stitch in which the thread is "woven" in rows along the grain of the fabric, with the stitcher reversing direction at the end of each row, and then filling in the framework thus created, as if weaving. Darning is a traditional method for repairing fabric damage or holes that do not run along a seam, and where patching is impractical or would create discomfort for the wearer, such as on the heel of a sock.
I have to admit that I do not darn. If my sock gets a hole, I throw it out, but I can sure remember my Grandmas darning and patching.
Even if I don't darn, I own three darning eggs.
Here is a photo of the darning process from Make: Craft blog.
There is even a tutorial on the above link on "how to darn" and more photos of darning.
I titled my day 20 nostalgia post "The Lost Art of Darning". But maybe I should ask first, do any of you darn?