Demonstrations may show what you know, but experiments show how you know it. Bench scientists tend to be skeptical. That's a valuable if undervalued mindset to impart to students. This skepticism generates creativity. Maybe the cream would have set spontaneously, without shaking, had we just let it sit.
Creativity generates competing ideas, and ingenuity finds ways to test the competing ideas.
Shake one, but not the other. But if only the cream in the shaken canister turns, while the cream in the other remains unchanged, you have tested and found that shaking is essential. You can contemplate not just one item, but you can simultaneously compare two things. In a recipe, you just follow one line of instructions. In an experiment, you do two things in parallel, two things that differ by only one factor, so any difference in result can be attributed to the difference in treatment.