One of my favorite activities to do with my young children is mixing baking soda and vinegar. First, it’s easy—I almost always have both in the house. Second, it’s harmless. You won’t find it staining a favorite shirt in next week’s laundry (like slime) or stuck in your shoelaces next Valentine’s Day (like glitter).

But most importantly, I love seeing the big reactions on their little faces. Something so simple and ordinary becomes extraordinary. When I did this experiment with my four-year-old this month—for the first (and then second, third, and fourth) time—it hit me: this was my last time seeing her very first excitement. Soon, she’ll be too old to be fascinated by a baking soda plate and a vinegar dropper. The bubbles and the stinky smell won’t be enough to capture her attention.

So instead of rushing through it—of giving in to the exasperation of “5 more minutes please?” when I had other plans—I chose to embrace this “last first.” I pushed back our schedule, poured in more baking soda, let her add a little extra food coloring, and refilled the vinegar so she could “see what happens.”

And when it was all done, I rewarded myself with an adult sticker (a latte) for showing exemplary patience and flexibility. Because, honestly, I did a good job too.