How might one explain bipolar disorder to a child?

Bipolar explaination. It depends on the age. Bipolar disorder affects primarily one's mood. For kids, that often results in behavioral disturbances. I would explain to a young child, that he or she has a problem with: "taking charge of yourself, " as that would seem to be the case overall. Ask your child what you can do to help him or her take better charge of themselves. Good luck.
How old is child. Younger they are, the more concrete examples like storms and strong weather changes that take over your feelings. Others have used a roller coaster metaphor. Or mood thermometer with different colors indicated heat of rages, yellow for between and blue for sad and asking where are you at this time. My Bipolar, Roller Coaster, Feelings Book by Bryna Hebert (Mar 31, 2005- great reviews.
Depends on age. 3-7 year olds need a very simple explanation. Older children or adolescents should participate in discussions of their condition with the doctor. Together, parents, physician, counselor & patient form an interactive team to educate, discuss, treat & improve the life of a bipolar child. Use a volcano metaphor as a simple form of explanation. One explodes for no (or minimal) reason then cools down.