If you mean like AA or AAA, nothing. Absolutely nothing happens. If they both have an electrical current running, I don't know, it might be dangerous.
Dead battery -will no longer charge.
The battery contains Sulfuric acid and this can burn the skin. The best treatment is to wash the effected part in cold water, much the same as any type of burn.
changes color
It changes color.
Tordon is a herbicide and when it comes in contact with skin, the skin will become irritated.
It changes color
Nothing
When two neutral electrical charges come in contact, there is no transfer of electrons between them. They will remain neutral and there will be no repulsion or attraction between them.
Contact Metamorphism: When magma comes in contact with other rocks it will heat them and change them into new rocks. This often happens around an igneous intrusion.
it change color
It changes color
It'll turn purple.