何で
なんで · nande · JLPT N3 · adverb
Why
何 (what) + で (by/with) = by what? = why; what for
で marks the means or cause, so 何で literally asks 'by what?' or 'because of what?'. Casual なんで is the everyday why; the same spelling read なにで asks 'by what means', as in なにで来た? (how did you get here?).