Following this pair of tweets about water:

The obvious question is, at what point are the two numbers the same? Or,

If you put all the Earth’s water into containers of the same size so that each container carries as many atoms of water as there are containers, how big is each container?

Facts

Working

We want this equation to hold:

\[ \textrm{atoms in }1l \textrm{ of water} \times \textrm{volume of container} = \frac{\textrm{litres of water on Earth}}{\textrm{volume of container}} \]

Rearrange that to get an expression for the volume of the container:

\[ \begin{align} \textrm{volume of container} &= \sqrt{\frac{ \textrm{litres of water on Earth} }{ \textrm{atoms in }1l \textrm{ of water}}} \\ &= \sqrt{ \frac{1.386 \times 10^{21}}{1 \times 10^{26}}} \\ &= \sqrt{1.386 \times 10^{-5}} \\ &= 0.00372290209\;l \\ &\approx 3.723\;ml \end{align} \]

Conclusion

The container is really, really tiny — just over half a teaspoon!