火花塞

• 火(fire (pictographic))ASSOCIATIVE

The character 火 depicts a flame bursting with fire sparks radiating out on both sides.

• 艹(grass; plant)RADICAL
• 化(change; transform)PHONO-SEMANTIC

A flower is grass and trees transformed into the most brilliant and beautiful appearance.

• 宀(roof; cover)ASSOCIATIVE
• 土(earth; soil; land)RADICAL

Stuffing earth under a roof to seal all the gaps is what blocks and fills means.