Bottles are gone??

Ok, this afternoon I get up (I worked last night) to find that the girls had somehow found an old bottle (they haven't had bottles since they were a year old) and they were fighting over it. After waging the constant battle to get them to keep their straw cups down instead of picking them up like a bottle to drink, I couldn't get them to lift the bottles up, they kept trying to drink out of them like straws. Silly, silly girls. Needless to say, the bottles will once again mysteriously disappear.