ying-yang
A ying-yang is a circular symbol split in half so that each half looks like a sideways tear-drop. The two sides are labeled Ying and Yang. Ying is white with a circle of black in the bulb of its tear and has the point of the tear up. Yang is black with a circle of white in the bulb of its tear, its point is down. 
A ying-yang symbolizes the belief that there is two sides to everything. The light without dark, no day without night, happiness without sadness, good without bad, ect..
It also shows that nothing is purely good or bad or so on and so forth. The two pieces on their own are half, together they are whole, and balanced. the Ying side represents good/light/day ( pretty much anything positive) and Yang  is pretty much anything negative. Ying, in a ying-yang, is always on the left side.
The twins act just like a ying-yang, one is always causing trouble, and the other is always so nice a sweet.