Trees in a mug. A fantastic image of the fractal patterns forming between two liquids of different densities. It was formed from adding to icing sugar to food colouring. This looks like a classic example of the Saffman-Taylor instability, which occurs when a less dense fluid is moving towards a more dense one, although I’m not sure the respective densities of icing sugar and food colouring.
Source: Reddit - howabootthat Worth checking out the comments for some cool versions