If your child asks questions abruptly, answer them. The random learnings are effectively stored in their mind.
Random learnings are deep rooted than those done in the time assigned to study. For e.g. when you watch TV, children are really moved by the advertisements. They might not remember their jingles but will remember what their favourite characters quoted in the ad. Similarly, anything that comes to their notice spontaneously will be retained for a longer time. So try giving them unexpected tasks and questions while they roam in the house or you are on an outing.