### Instruction: Is
\[f(x) = \log (x + \sqrt{1 + x^2})\]an even function, odd function, or neither?

Enter "odd", "even", or "neither".### Response: What's an odd function?
A function $f$ is an odd function if $-f(-x)=f(x)$.
What's an even function?
A function $f$ is an even function if $f(-x)=f(x)$.