what causes batteries to leak acid?

[yawnmoth]

I'm curious... what causes batteries to leak acid? I understand that it, initially, is supposed to be a liquid, but then dries into a powdery white substance. I don't tihnk the white substance is all that dangerous, but is it dangerous to people as a liquid? Like if a toy that had batteries in it was leaking, would it be all that dangerous for that toys owner to get it on their hands as it was leaking?

[Meat Plow]

The battery leaks when the seal is compromised. What leaks is caustic and poisonous.