What is a science concept that we can teach preschoolers that's purely visual? We are trying to develop a scientific study to test preschoolers (3-5 years old) ability to learn a new science concept. The problem is that children that young can only remember concepts based on how they look but cannot remember any abstract information. So we are trying to find a science concept that can be taught to young children purely visually (feature based), that they can remember and then be tested on. What is a good example of this?

Example of a purely visual concept. (Differentiating species based on features):
Consider showing a visual representation of the Earth-Moon-Sun system: either with videos or with globes.


Show a globe of the Earth, that it's round but looks flat up close, and that it spins. 
Show that the Sun is also round, much larger than the Earth, but very far away, and looks like a circle from far away.
Show that the moon is also round, smaller than Earth, but not as far away (people have been there, the landing spots can be pointed to at night, and there are pictures of Earth from the Moon)
Show that the Moon orbits the Earth, that's why there are moon phases (notice how the lit side is always pointing towards the Sun) and the moon disappears every month (the Sun outshines the Moon when they coincide in the sky)
Show that the Earth orbits the Sun (this one is harder to intuitively demonstrate)
Show that the Earth axis has a tilt, and that's why there are seasons (might be stretch).
Then to really challenge them, you could even tell that some people don't believe any of this, and instead they believe that the Earth is flat and argue with each other about various details of how it's flat.