The sand under the foreskin myth is an obvious invention of circumcised men to justify circumcising other men. It defies all logic and reason.
Supposedly, the mechanics of the myth is that sharp sand grains get under the foreskin and in the foreskin rubbing back and forth between the foreskin and glans, it abrades or cuts into the surface of the inner foreskin or glans causing abrasions or actual cuts which then become infected and thus requires circumcision.
The actual fact is that the inner foreskin does not move against the glans so there is no friction or abrasion possible. When retracting, the foreskin rolls inward to outward in a friction free movement. Heck, you could pack the foreskin full of sharp sand and this wouldn't happen. More over, the man could simply slightly pinch the tip of the foreskin retaining some urine inside the foreskin to flush any sand out as he urinated.
This makes one wonder why circumcised men want other men circumcised so badly that they will invent these myths and why others are so eager to accept them and perpetrate the myths.
Frank