In my childhood I collected coins. It was my hobby. During WW II copper was a very valuable metal used in the war effort. The U.S. Mint was asked to produce a steel penny so in 1943 they did just that. One day I ran across 3 steel pennies in mint condition. I bought them, went home, and put them in my book for pennies. Six months later I found another penny I was looking for and put it in the book. For curiosity I took out the steel pennies and was shocked. The front was still beautiful but the back had reacted to the paper in the book. Each penny was eaten and green. It had rotted. Jesus said in Matthew 6:19-20,
Do not store up for yourselves treasures on earth, where moth and rust destroy, and where thieves break in and steal. But store up for yourselves treasures in heaven, where moth and rust do not destroy, and where thieves do not break in and steal. For where your treasure is, there your heart will be also.