When I feel like my life is in a period of low spirits, I often draw a little sun on my left knee. I will not get it tattooed there, because I like it to change so that it can best represent and alter my mood.
Other things that have been on my left knee lately include:
A pink, pierced heart, when my knee needed some loving.
A green, glowing brain, when I needed extra help on my psychology exam.
A blue cat, dreaming of fish. This was drawn by a friend, but still managed to reflect my feelings. My knee is quite magical that way.