I think that the hardest topic to teach to elementary students is anything that is smaller than what they can see with their naked eyes. For example, my students struggled when learning about atoms because they could not grasp the concept that their desks were made of trillions (or more) atoms. My students have also struggled with learning about cells in plants and animals. Although we used a microscope to view both plant and animal cells, my students did not seem to fully understand what a cell is or the fact that they are made of cells. Although I do not think that most elementary students are developmentally ready to fully understand atoms and cells, I support inducing such topics to students so that they are challenged to critically think about what is happening outside of their own vision or world from a young age.