"Snow globes are not permitted to be carried through security checkpoints," said Transportation Security Administration spokesman Dwayne Baird.Even a small snow globe with less than three ounces of liquid may be confiscated because security personnel have no way of measuring it. You can pack a snow globe in your checked luggage, but be careful to pad it well, or you may end up with wet clothing by the time you reach your destination. Link -via Metafilter
The reason is that the globes contain liquids, and TSA rules say that only liquids, gels or aerosols in containers of three ounces or less are allowed through security in carry-on bags.
(image by Flickr user revrev)