Dont put sand in your underwear

Im not sure why this is such and exciting thing but lately my boys have decided it is very fun to put as much sand as you can in your underwear.   Sometimes they stay in the sandbox and sometimes they walk all over the yard, or even jump on the trampoline while the sand is falling all out.... but inevitably I end up with cups of sand on my carpet.  I think this is one of those things you need to write down so you can laugh about it later, but i really get sick of asking them not to put sand in their underwear.   Then later of course you have the sand in the butt crack that is not very fun to remove.  At this point hindsight being 20/2o I think i would have opted not to have a sandbox with a ton of sand in it.  I am wondering how long it is going to take for every grain of that sand to eventually make its way into my house.