The experiment with fresh and frozen pineapple versus canned pineapple highlights the importance of enzyme activity in relation to heat. Fresh and frozen pineapple contain bromelain, an enzyme that breaks down proteins. When used in Jello, it causes the Jello to become liquidy because it denatures the protein in the gelatin. On the other hand, canned pineapple has been heated during processing, which denatures the bromelain enzyme and renders it inactive, thus not affecting the Jello.
Based on this analysis, the most appropriate lesson from your observations is:
They work better at some temperatures than others.
This indicates that the heat from the canning process inactivated the bromelain enzyme, demonstrating the effect of temperature on enzyme functionality.