Pastry layers puff because trapped steam expands the dough

To create the signature lift of a Danish pastry, bakers use a technique called lamination to fold cold butter into dough exactly 27 times, creating hundreds of distinct, alternating layers. When the tray enters a 200-degree oven, the water trapped within the butter flash-boils into steam. Because the fat layers act as a waterproof barrier, the steam has nowhere to go but up, physically shoving the dough layers apart until the pastry swells to double its original height.