Children are adept at recognizing similarities and differences between their toys and the real objects the toys model. They can be led to discuss those comparisons and systematize their observations. Toy cars may demonstrate gravity very well, but won't show much about propulsion. Balloons show different principles, depending on whether they're filled with air or helium, and depending on how heavy and permeable the balloons themselves are.
Children have seen that some things change --- the puppy or flower grows, the snow melts --- and other things stay the same --- the soil in the flower bed lies still unless someone digs in it. These are major themes for science and categories for systematic observation. What is similar about the things that grow, and how are they different? Does the melting snow have any effect on the soil? The AAAS benchmarks encourage teachers to introduce, before the close of grade 2, ways to observe and analyze changes that may be too fast or too slow to observe directly.
Children have been experimenting and storing their observations from their infancy --- how long can you dunk different kinds of cookies into milk before they become saturated and fall to the bottom of the glass? But they haven't been recording those observations with any precision of measurement that can be shared with other observers. By the primary grades, they're ready to make those observations truly scientific through replication of results, and to collaborate with other scientists.
Primary-school students may have looked at the stars and concluded there were too many to count. They may have noticed that some are brighter and steadier than others. They can begin now to learn how to stand on the shoulders of giants who have counted the uncountable, whether too big or too tiny to take in at once, and to learn that their own observations are more powerful when combined with those of others who've used telescopes, microscopes, spectroscopes and other instruments to observe beyond the power of the naked eye.