Hot lava is a viscous fluid that, driven by gravity, moves along the Earth's surface. Intuitively, one attributes constructional properties to lava-it accumulates in volcanic landforms, compound lava fields and, in the end, entire mountains. On the other hand, there are also examples of the erosive power of lava: on Earth and especially on other planets in the Solar System, there exist channels incised by flowing lava. The origins of these erosive features have long been debated among volcanologists and planetologists. The dominant paradigm is thermal erosion, although it leaves many questions open. After the 2001 eruption on Mount Etna we found a lava channel whose features cannot be explained in the frame of thermal erosion. On the basis of our observations, we have developed a model for mechanical erosion that explains the main field observations, and opens alternative ways to describe erosion by flowing lava.