The problem states: "The major content of penny is actually zinc (97.5% w/w). Devise a simple experiment (no chemical reactions involved) to prove this fact."

Since we know that 97.5% of pennies are Zn and 2.5% is Cu, my thoughts were weighing a sample with 97.5 mg Zn + 2.5 mg Cu and then seeing if that weight is proportional to the weight of the penny. But I asked my professor this and he said there is a much simpler way... He simply told me: "How do you differentiate metals?" I haven't really figured it out... Any help would be much appreciated. Thank you!