Melt two metals together and the strangest thing happens: the alloy melts LOWER than either parent — at one exact recipe, the eutectic, it melts at a single sharp temperature far below both. That is why solder is an alloy. Slide the composition across the phase diagram, watch the two lattices fuse, and tap to pour them into one.
A real tin–lead phase diagram, computed live. Pure lead melts at 327°C, pure tin at 232°C, yet the alloy's liquidus DIPS from both sides to a single lowest point — the EUTECTIC at 61.9% tin, 183°C — below either pure metal. Slide the composition and the page reads the melting temperature straight off the two liquidus lines and flags when you are below both parents. The eutectic minimum, where two solids melt as one, is the exact, real content — two tempered into a substance neither parent could be.
Straight-line liquidus segments approximate the real (slightly curved) Sn–Pb boundaries, and the solid-solution regions at the edges are omitted; the numbers (327, 232, 183°C, 61.9%) are the real handbook values. The eutectic-below-both-parents fact is exact.