This is a very cute puzzle borrowed from the book Mathematical Circles (Russian Experience) by Dmitri Fomin, Sergey Genkin and Ilia Itenberg. It is a wonderful book for middle graders.
There are six sparrows sitting on six trees, one sparrow on each tree. The tress stand in a row, with 10 meters between any two neighbouring trees.If a sparrow flies from one tree to another, then at the same time some other sparrow flies from some other tree to another the same distance away, but in the opposite direction. Is it possible for all the sparrows to gather on one tree? What if there are seven tree and seven sparrows?
As always, please send your answers as comments within the blog (preferred), or send an e-mail to firstname.lastname@example.org. Please do share the puzzle with others if you like, and please also send puzzles that you have come across that you think I can share in this blog.
Happy flying on the trees!