  We present an analogy between two unrelated instabilities. One is caused by
the impact of a drop of water on a solid surface while the other one concerns a
tumor that develops invasive cellular branches into the surrounding host
tissue. In spite of the apparent abstractness of the idea, it yields a very
practical result, i.e. an index that predicts tumor invasion based on a few
measurable parameters. We discuss its application in the context of
experimental data and suggest potential clinical implications.
