If you want to have some fun, you can smoke pot while standing on the border and holding it in your left, Dutch hand. If you're really daring you swap it over to your right, Belgian hand, and congrats, you're an international drug smuggler. If a cop sees it and you're unlucky, I suppose he'll ask a few puffs from you - Yeah I saw that happen in real life, and that was in Brussels a.k.a Belgium and as far from the boundary as you can get.