We investigate some basic properties of the heart (K) of a convex set K. It is a subset of K, whose definition is based on mirror reflections of euclidean space, and is a non-local object. The main motivation of our interest for (K) is that this gives an estimate of the location of the hot spot in a convex heat conductor with boundary temperature grounded at zero. Here, we investigate on the relation between (K) and the mirror symmetries of K; we show that (K) contains many (geometrically and

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... isically) relevant points of K; we prove a simple geometrical lower estimate for the diameter of (K); we also prove an upper estimate for the area of (K), when K is a triangle.