We drove down the highway on a gorgeous winter day, thunderheads scattered across the blue sky, my wife beside me, our five-year-old son in the back seat.

“Daddy, what are clouds?” he asked.

“Condensed water vapor,” I said.

“Huh?”

“Condensed water vapor. Matter has three states: solid, liquid, and gas. There’s also a fourth state called plasma, but I don’t really understand it. Water vapor is a gas. It condenses into liquid at the dew point.”

“Drool pint?”

“Dew point.”

“Drool pint?”

“Close enough,” I said, excited he’d finally shown an interest in science after my relentless prodding. “You also need to know about the lapse rate: air gets colder as you gain altitude. At least until you reach the stratosphere, where ozone absorbs sunlight and warms the air, creating an inversion layer. That’s why those thunderheads have an anvil shape. So if you graphed temperature versus altitude,” I drew a little zigzag in the air, “the lapse rate would look like this. Get it?”

He stared at me blankly.

I felt flummoxed, needed to dumb it down. “Okay, just remember two words: evaporation and condensation. That’s all you need to know. Along with dew point. And lapse rate.”

He looked out the window for a moment, eyeing the thunderheads. Then he turned back to me. “Daddy, what are clouds?”

I rubbed my neck in frustration.

My wife glanced back. “Tiny drops of water, honey, floating in the sky.”

He nodded. “Oh, okay. I get it.”

“Thanks,” I muttered in relief.

“My pleasure,” she whispered. “Temperature graphs are sexy.”

“Really?”

She smirked and patted my hand. “Sure, babe, sure.”