Britain is gearing up for a chilly weekend, as forecasters predict a bad cold snap. Even the local birdbaths are at the ready: as The Sun reports, residents have reported a rash of ice spikes, sharply pointed frozen spears that only form under specific conditions.

A good ice spike requires three things: contained water, a slight breeze, and temperatures that are cold but not too cold (around 7 degrees Fahrenheit is best). As the outside layer of water freezes, it expands, forcing the excess, liquid water up through the top layer of ice.

This water freezes as it escapes, eventually forming the spike. The breeze, coupled with a particular type of initial crystallization, encourages movement upward and helps with the jaunty angle.

Dangerous conditions aren’t necessary for ice spikes—they’ve been known to show up in household freezers, in the cube trays. But they are fairly rare, and certainly ominous—although they do make a good perch for thirsty, foiled birds.

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