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Every chemical element has one or more radioactive isotopes.
For example, hydrogen, the lightest element, has three isotopes, which have mass numbers 1, 2, and 3.
Organic materials typically contain small amounts of radioactive carbon and potassium.
A swimming race illustrates the simple principles involved in measuring time.
This swimmer is competing in a 1,500 metre race and we have an accurate, calibrated wristwatch.
Radioactive isotopes of radium, thorium, and uranium, for example, are found naturally in rocks and soil.
Uranium and thorium also occur in trace amounts in water.