My GF and I were out riding today and a guy stopped as he passed us when he noticed one of our mini pumps. He said another guy was looking for a pump around the corner, and sure enough there was a guy with an older Haro mountain bike with a flat tire on the front. He said he'd been riding for a while, and noticed the tire was flat after a while. He hadn't been riding in ages, but said the tire did have air in it when he left home. He was sure it would hold air after we pumped it up, and sure enough it did. He went the same way we were going for a while too, so I knew it held air for a while. How could the tire loose air while he was riding and then hold air after we pumped it up?