Parents love to warn each other about the next difficult phase to come, and nothing is quite as challenging as a small human who is testing your boundaries. But there's a lot going on behind that crumpled up red face, that defiant "no!" and the endless tears. Dr Laura Jana is a paediatrician and author of The Toddler Brain. She tells us what's going on inside our toddlers' heads, and how to decipher it so we can handle the more challenging moments better.