An archetypical symbol, found in nearly all cultures. In general, circles tend to exemplify the Earth Mother as opposed to more masculine gods. They are used to help represent balance (yin-yang), the sun, the moon, completeness, or even reincarnation and infinity (Ouroboros).

There are lots of circular toys, like yo-yos, inner tubes, hula hoops, and frisbees.
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Offtopic: This writeup exemplifies my life. It used to be a writeup for the wrong node (see: Rolling), but I decided to delete it and meaningfully contribute to the discussion about circles. Kind of like when I give the nice guy behind the counter the wrong order or when I don't get what I ordered and I just take it anyway to try something new.