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Droppings

what-if.xkcd.com · 474 words · saved by 1 readers

What If? 2: Additional Serious Scientific Answers to Absurd Hypothetical Questions is out! Order here! If you went outside and lay down on your back with your mouth open, how long would you have to wait until a bird pooped in it? –ADRIENNE OLSON Answer: Period = 1 Frequency = 1 300 billion birds 4 𝜋 × Earth radius 2 × 1 poop bird hour × 16 hours day × 1 mouth poop × 15 cm 2 mouth = 195 years Unit cancellation is weird. Of course, that equation makes a few simplifying assumptions. It assumes there are 300 billion birds in the world. This number comes from the best-titled source ever—“How many birds are there?”, an actual academic paper published in 1996 in the journal Biodiversity and Conservation. The equation also assumes birds are randomly distributed across the Earth’s surface (they’re not) and that they poop an average of once an hour at a random location (they don't, although having owned a bird, I can tell you that’s probably more accurate than you might think). In the real w

If you went outside and lay down on your back with your mouth open, how long would you have to wait until a bird pooped in it? –Adrienne Olson Answer: \[\text{Period}=\frac{1}{\text{Frequency}}=\frac{1}{\frac{300 \text{ billion birds}}{4\pi\times\text{Earth radius}^2}\times 1\frac{\frac{\text{poop}}{\text{bird}}}{\text{hour}}\times16\frac{\text{hours}}{\text{day}}\times 1 \frac{\text{mouth}}{\text{poop}}\times 15 \frac{\text{ cm}^2}{\text{mouth}}}=195\text{ years}\] Unit cancellation is weird. Of course, that equation makes a few simplifying assumptions. It assumes there are 300 billion birds

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