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The Common App gives each activity 50 characters for your position, 100 for the organization and 150 for the description, spaces included. Draft all ten here and see every limit as you type.
Led, built, taught, organized. Say what you did, not what the club does.
People, hours, events, money raised. Only numbers you could stand behind.
Fragments and semicolons are fine. Articles and "I" usually are not needed.
Put first what took the most time, carried the most responsibility or matters most.
The full guide covers what counts as an activity and the mistakes readers see most: how to write the Common App activities list.