induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
1 The damage that a Ground type attack normally does is multiplied by 0 when it is used against a Flying type pokÃ©mon.  The susceptibility of Fire types against Water attacks is 2, because Water type attacks are strong against Fire type PokÃ©mon. The damage that a Water type attack normally does is doubled when it is used against a Fire type pokÃ©mon.