Sally is a sleepwalker. Each night she gets up without knowing it, puts on her slippers and coat, and wanders around her village for an hour or so, then returns home, climbs upstairs, and gets into bed. The only reason Sally knows she sleepwalks is because her coat has a hole in its pocket so she drops things as she sleepwalks.

One day Sally sleepwalked with her coat pocket full with pennies. When she woke up in the morning and walked around her village, she found a penny at each intersection. When she had picked up all the pennies and returned home she suddenly realized something interesting... she had not walked on any street more than once! How did she do it?

Sally's house is at the top.

A student at Sundance Elementary School.

Extensions:

• Sally can choose her first street to walk down in 3 ways. Her second, third and fourth choices are between two streets. Can Sally pick up all her pennies no matter which decisions she has made?

Credits:

This problem was invented by the great mathematician William Rowen Hamilton.

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