I use them to store rice, popcorn, nuts, sunflower seeds, herbs, spices, etc.
I often put them in the freezer as well, because most grains and nuts freeze very well and the good seal that the canning jar lid provides very effectively prevents freezer burn or that funny freezer taste that things can get.
That is great advice. It makes sense a length of red yarn would be easier to see than a little dot of red nail polish.