Imperative

"I need you to help me with something."
He looked at her curiously. "With what?"
"Something important. Come on!"
He let her lead him by the hand, down the stairs, out the door, into the chilly, not-quite-Spring air. Fat snowflakes danced lazily about them, and she laughed as she tugged on his arm.
"Here." She let go, reached down, and packed a quick snowball. "It's imperative that you help me build a snowman."
"Imperative?" He smirked at her, good-naturedly.
"Yes. Imperative. Necessary. If you don't, the sun will turn to ice and the Earth burst into flame."
It was his turn to laugh. "Well, then." He packed a ball and started rolling. "To save the world."
She threw her snowball at him.