Paper trash is a trash that promotes recycling since it only allows you throwing in paper. But if you throw in aluminum paper a sensor will detect it and tell you it doesn't belong there by showing you an X. But when you throw in a paper it shows tick telling you that it belongs there. The trash tells you that by knowing which is the amount of electricity every material has. Since the paper is not conductor it will show a very low value, but in the other hand the aluminum paper is conductor and it will show a higher value.