Those of us more accustomed to lessened visual acuity have learned that quarters have a serrated edge and nickels are smooth on the edge. You don't even have to take them out of your pocket to tell which you have.

They are sufficiently similar in size and weight that rigorous scientific tests in the lab a few decades ago showed that squashing a nickel between two solid steel plates at 16,000 pounds load would make the nickel look like a quarter to the vending machines down the hall, but I believe the machines have improved discrimination now.

Even then, coin-pay telephones could tell the difference, but there aren't any of those still in existence so the knowledge of how to turn nickels into quarters is totally academic and useless even if it had worked for the phones.

John