How do I explain an algorithm to my younger sister?

Think about how you tell her to brush her teeth. You don't just say 'brush.' You say: 'Pick up the brush, put paste on it, scrub the top, scrub the bottom, rinse.' That set of instructions is an algorithm.

It's just breaking a big goal into tiny, tiny steps that even a baby (or a computer!) can follow. I've spent years doing this for physics simulations—it's all about the details.