Once upon a time I briefly worked for an oil company in Cleveland. An older material scientist (whose name escapes me) once showed us interns a neat trick in the lab. He melted some Gallium metal (it melts in your hand) and showed us how a sheet of Aluminum will absorb Gallium (like dissolves like even liquid metals). The older gent, who was a British ex-pat, explained that during WW II, the RAF feared that the Germans would sneak over and sabotage their planes. In those days, aircraft were often made of unpainted Aluminum.
Here is a video showing how Gallium wrecks an aluminum coke can. Imagine some nefarious kraut doing that to an airplane's wings.
Actually, Aluminum is not absorbing--rather, Gallium is invading.
The reason that Gallium liquifies is related to why Copper is so ductile and bears repeating: if you give an atom a perfectly filled sub shell along with one extra electron (the 31st one), the metallic bonding will be weak and non-directional. It all makes perfect sense to me.