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Major delays and crazy footage followed a large water main break in Prince George’s County early Monday. The cause of the ruptured pipe is not immediately clear, though pipes often burst in the winter because the water expands. Except…The List thought molecules were supposed to contract in cold? Cold air contracts. Hot air rises. E=MC2. The more The List tried to recall high school chemistry, the more baffled she became. She turned to local plumber Malcolm Coleman and American University chemistry professor William Hirzy to explain why pipes burst in the winter.

1. Water behaves strangely under cold temperatures

   Does cold water expand or contract? Both, says Hirzy. As temperatures drop, water molecules contract. But water has a maximum density of just above freezing, and once the temperature hits around 37 degrees, it reverses course and expands. This strains pipes.

2. A burst pipe often starts from a leak

   Coleman explains that underground pipes stay constantly warm if they’re deep enough, but the cold winter water running through them strains them, creating cracks or leaks. The water doesn’t have to actually freeze — pressure can eventually force a crack to rupture.

3. A burst pipe can result from actual freezing if the conditions are right

   As Hirzy puts it, flowing water will continue to flow, unless the temperature is extremely cold. But if a pipe is close enough to the surface of the earth and there was low water demand during the night, it is possible for water to actually freeze in the pipe and burst.

4. Lead and age exacerbate the problem

   Oxygen in the water causes lead pipes to rust, further weakening the pipe. To prevent rusting, Hirzy explains, a block of some other element, like magnesium, might be in the system. (The magnesium oxidizes instead of the lead.) But in an older pipe, when the magnesium is gone, the lead is vulnerable again.

5. Pipes need to be winterized

   Coleman recommends insulating your pipes as well as shutting off your outdoor spigot and letting the pipe drain.

6. Be vigilant about leaks

   If your water bill goes up or your water pressure goes down, you might have a leak, Coleman says. (Some municipalities, like Montgomery County, have sensors on their pipes to monitor stress, but that’s not an option for residential pipes.) Repairing a leak in a house pipe might be $200 to $300, but repairing a burst pipe can run into the thousands.