Here's why:
* Spondee: A spondee is a metrical foot in poetry that consists of two stressed syllables (e.g., "BREAK-FAST").
* Pyrrhic: A pyrrhic is a metrical foot that consists of two unstressed syllables (e.g., "a-BOUT").
So, while a spondee is heavy and emphasized, a pyrrhic is light and unstressed. They are direct opposites in terms of their metrical structure.