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Mile

Noun - A unit of linear measurement equal to 5,280 feet, or 1,760 yards (approximately 1.609 km).

Stone

Noun - The hard, solid, nonmetallic mineral matter of which rock is made, esp. as a building material.

Verb - To throw stones at someone.

Milestone

Noun - A stone set up beside a road to mark the distance in miles to a particular place.

Also an action or event marking a significant change or stage in development.

Example Sentence:-

" Emma Watson said that her 21st birthday was a "milestone" in her life."