Julie finds a snail on the sidewalk and wants to know whether or not the snail moves throughout the day. She places a single mark on the sidewalk next to the snail. What will Julie use the mark for initially?

The mark gives Julie a fixed location to compare the snail’s position with later. If the snail’s position changes relative to that mark, she can tell that it moved, so the mark acts as a reference point for detecting motion.

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