Children learn best when learning is firmly rooted in experience of the real world.
That is simply because humans learn by processing their experience of the real world presented to them through their senses. Therefore, across the grades, new concepts are taken to children with a strong grounding in real-world experience.
Grade 1 children learn that numbers are about quantities of real things by working with counters. The same children learn about fractions in Grade 4 by working with wholes and parts of delicious food items that they then relish. In Grade 4, they also learn Geography by actually making maps by surveying regions to be mapped and representing their locations and spread on paper. Science, which starts in Grade 6 employs the phenomenological approach where concept formation proceeds from deep experience of natural phenomena.
Wherever possible and necessary, learning starts, in this manner, with experience of real world things and phenomena. Children thus develop their concepts and understanding in an organic manner. The living concepts thus formed grow with and in the children, not just stay there like unused books gathering dust on a library bookshelf.