In lab, we purified acetanilide through recrystallization. We did not have have time to test the melting point to check the purification, so we put the recovered crystals in a 50ml beaker, covered the top with parafilm with a few holes poked in the top, and left them for the next lab. When we left, everyone had crystals in their beakers. When we returned a week later, only one group had crystals left in their beakers. How could this have happened? The beakers were not messed with by any other class or person working in the lab. Any thoughts?