Breaking down complex words into smaller meaningful units (e.g., "unhappy = un- + happy"), known as morphemes, is vital for skilled reading as it allows readers to rapidly compute word meanings. There is agreement that children rely on reading experience to acquire morphological knowledge in English; however, the nature of this experience has remained unclear. We quantify the morphological information in 1200 popular children's books and offer the first concrete description of how readers may learn affix morphemes through real-life text input. Our account considers the realities of morpheme presentation in natural language, such as the low number of distinct words in which affixes appear and the fact that their identification often requires specialised linguistic knowledge. This theory further accounts for the challenge posed by spelling patterns that may lead to incorrect morphological parsing. We conclude by exploring the implications of our findings for instructional programmes in morphology.