Parts per million, billion, trillion explained in simple terms

Your water is tested extensively to ensure it is safe. Water quality is usually reported in parts per million, billion or trillion. What does this mean in terms we can understand?
• Parts per million (ppm)
You clean a 10-gallon fish tank and 1 drop splashes out.
You stock up with 10,000 boxes of tissues, each with 200 sheets. You use two sheets. That’s 1 ppm.
• Parts per billion (ppb)
You inherit $10 million. In counting it out, you discover 1 cent is missing.

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