burn
爇 centers on burning: setting something on fire or being consumed by flames.
爇 combines 艹 (grass) with 熱 (heat), suggesting burning grass or setting fire to vegetation. The exact historical development is uncertain.
Grass 艹 on top of heat 熱: imagine dry grass catching fire from the intense heat below, burning fiercely.
For ゼツ, picture a zesty flame burning the grass: zesty -> ゼツ, and the fire is hot and bright.