Find the maximum value of $10^x - 100^x,$ over all real numbers $x.$
Let $y = 10^x.$  Then
\[10^x - 100^x = y - y^2 = \frac{1}{4} - \left( y -  \frac{1}{2} \right)^2.\]Thus, the maximum value is $\boxed{\frac{1}{4}},$ which occurs when $y = \frac{1}{2},$ or $x = \log_{10} \left( \frac{1}{2} \right).$