Typically in these situations, I'd ask him "what do you think will happen if we plant them?" I either turn it into a science experiment or I break the news as to why this wouldn't work. However, tonight I smiled, grabbed my camera, and said "let's plant them."
Somewhere in this grass is a dozen popcorn kernels.
I'm quite certain there will be hell to pay in a few days when he inquires about why his popcorn kernels aren't growing into popcorn. I'll break it to him then that those kernels aren't going to make him a yard full of popcorn.
Until then, I'll smiled when I notice popcorn kernels scattered throughout the front yard. I'll think about how excited he was when he got his idea. I'll remember the simplicity of being a kid. When throwing popcorn kernels in the yard in hopes that they'll grow massive amounts of popcorn was just as exciting as Christmas morning.