Scientists have developed a new way to watch brain cells in action. The technology uses a tiny microscope that can be attached to the head of a mouse and capture images deep inside the brain, under different angles and depths. The result is a 3D image that shows the neurons turning on and off while they communicate inside the brain. This approach is faster and more effective than other currently available methods and brings neuroscience closer to the dream of recording millions of neurons working together in a freely moving organism.