Strange safety tip !, this one not really a safety tip !, ( i maybe flawed thinking about how frozen science works !, i try to experiment it maybe not make a difference !) but if you work in a hot climate and rely on frozen water bottles to keep cool !, maybe put each bottle in freezer at different times so that the melt rates will not be the same as if you put multiple bottles all at once in freezer , if you put them in at different hours your frozen water maybe last a little longer !