In schools, subjects are fitted into neat boxes, regardless of overlap. For example, social history and geography may cover some of the same material - when I was at school I found that traditional farming methods were studied in both, but different teachers, classrooms and times of the day all serve to separate the information in a child's mind. Lessons learned in one subject stay in the box labeled 'Geography' and are not easily applied to history, which has its own neatly labeled box.
In unschooling, there are no boxes. You live your life, absorbing information without restricting it to certain subjects.
Yesterday Small and I baked a vegan chocolate cake recipe. We are not vegan, but it is a tasty and reliable recipe that is very popular in our house. The ingredients are measured in cups and it is Small's job to add them to the mixing bowl. Working in cups gave me the chance to explain simple fractions to him - we need 1 1/4 cups of flour, so for the quarter we split the cup into four parts and fill one of those parts with the flour.
The fun in this recipe is that the replacement for eggs is a vinegar and bicarbonate of soda mix. When these ingredients are added they fizz, just like the classic volcano experiment using the same ingredients, but at slightly less spectacular levels - I don't want the cake mix fizzing out of the mixing bowl!
So in baking a cake you could say we had covered Home Economics, Mathematics and Chemistry. As far as Small is concerned we just baked a cake, anything he learned was just incidental. And I am more than happy for him to look at it that way. Who needs boxes anyway?