Thought I'd lift you up a bit so you can see inside. It's important when working with green wood that what you are making is fairly even in thickness. Thin walls help the bowls dry quickly and evenly, encouraging them to find new organic shapes as they dry. If a form is thick in some areas and thin in others the moisture will escape the thin areas first and those spots will become harder and more brittle, while the neighbouring material continues to release moisture and move, usually resulting in cracking. So, the even consistency of these bowls ensures that the tension and strain the drying process puts on their original shapes results in warping rather than cracking. Green wood can cause endless headaches if not worked correctly, but it's fairly well behaved if a few basic principles are followed. Splits that weren't already present in the tree will usually only occur because the wood is left too thick, has tried too quickly, or wasn't an even thickness.