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?

Julie can compare the snail’s position with the mark over time. If the snail changes position relative to that fixed mark, she can detect that it has moved. The mark is therefore a reference point for detecting motion, not a unit for measuring speed or distance.

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