As nightfall approaches, a small group of students takes to the streets around College Park armed with hammers and thin plastic tubes. They measure power lines in residential districts with a laser-guided tool, hammer the tubes into the ground and walk on. Sometimes puzzled, residents stare from their windows, wondering if they should call the police.

But the students' strange behavior has a purpose behind it. Fending off curious city residents, the students are constructing an eruv - a "metaphysical wall" that allows Jews to carry items on the Sabbath. Jewish law prohibits worshippers from carrying anything during the Sabbath in any area deemed public. An eruv acts as a boundary around a public place, and allows the enclosed space to be considered private and therefore proper to carry items in.

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