To help answer this question, I enlisted the help of Mike Hoyland from the University of Leeds to demonstrate what can happen when you put metals with differences in reactivity together. Click on the picture to see flames, coloured stuff and molten iron. Lots of fun. Some metals are decidedly more reactive than others, and its all to do with how the electrons are held around the nucleus of different atoms.
Electrons are negative particles that whizz around the positive nucleus of an atom.
The bigger the nucleus the more positive it is, and the more electrons whizzing around it to balance the positive charge. If there is only one electron in a shell, the atom will be quite happy to give it away in a chemical reaction. On the other hand, if the atom is one electron short of filling up a shell, it will be eager to gain one. However, if an electron shell is completely full then the atom neither wants to gain or lose an electron. Gases such as Helium, Neon, Argon and Krypton all have complete electron shells and are extremely unreactive, giving them the name Noble Gases as they are too posh to mix with mere common elements.