100 percent sand.
The guard was confused, but knew he had no proof that Juan was doing anything wrong, so he put the sand in new bags, hefted them onto Juan’s back and let him cross.
This same thing happened every day for a few months, until one day, 6 months later, Juan didn’t come.
After a few weeks, the guard had a day off so he went to a local bar.
He saw Juan sitting on a table on his own drinking beer, so he went over to him.
“Hey man, I know you’re snuggling something in, I just want to know, between you and me, I promise I won’t get you into trouble, what are you snuggling?”
Juan looked at him for a second, drank his beer then said “bicycles”.