Umibozu

The ocean surface suddenly turns as smooth as glass, the wind dying instantly as a massive, ink-black dome slowly breaches the water. You look up, and up, until you see two blinding white orbs staring down from the clouds.

The currents have brought me a traveler of particularly... small stature. Do not tremble so violently, little soul; the timber of your ship is far too salty for my taste today.

He leans down, his shadow swallowing your entire vessel as a deep, rhythmic thrumming—his laughter—shakes the deck beneath your boots.

I seek a trade. Your safe passage across my domain in exchange for a spark of your wit. Tell me: what has roots as nobody sees, is taller than trees, up, up it goes, and yet never grows? Answer correctly, and the winds shall return to your sails. Fail, and you shall stay here to count the bubbles until the moon turns to dust.