Ishaan says Mount Everest is the "tallest" mountain on Earth, no question about it

Why

Ishaan's answer is correct for the measurement everyone actually uses on maps and in the news — Everest's summit sits higher above SEA LEVEL than any other mountain's summit, and that is genuinely the standard, most useful way to compare mountains. But 'tallest' can also mean something else: how far a mountain rises from its own true base to its peak, regardless of where that base happens to sit relative to the ocean. Hawaii's Mauna Kea is a volcano whose base sits on the Pacific Ocean floor, roughly 6,000 metres below sea level — measured from that true base to its summit, Mauna Kea rises about 10,200 metres in total, over 1,300 metres MORE than Everest's roughly 8,850 metres above sea level. Test it: Everest only 'wins' because its base — the surrounding Himalayan plateau — already starts very high above sea level, so it does not have as far to climb from its own base as Mauna Kea does from the ocean floor. His sister is right: by sea-level elevation Everest is the highest point on Earth, but measured true base-to-peak, Mauna Kea is actually taller — 'tallest' genuinely depends on which measurement is meant.