Practically everything is porous, but some articles are more so than others. In the case of things which are only slightly porous, it requires tremendous pressure to reveal their porosity and, for ordinary purposes, we might say they are non-porous. A piece of sheet tin, for instance, might come under the heading of non-porous bodies, but a postcard exhibits a considerable degree of porosity.
We can prove this by performing the following experiment. Fill two tumblers with boiling water and stand a postcard on one and a sheet of tin on the other. Over each tumbler stand another tumbler, but put them upside down. In a few minutes the tumbler inverted over the postcard is filled with steam, but that over the tin is free from steam. Clearly, the steam has worked its way through the card, showing that it is porous; but as the steam has not worked its way through the tin, we may take it that the metal is almost non-porous.