Q.Evaluate $\log_{10} 1000$.

A common logarithm (base 10) of a power of 10 is simply that power.

Since $10^3=1000$, $\log_{10}1000=3$.

$1000$ is a power of $10$: $10^3 = 10\times10\times10 = 1000$. By the definition of logarithm:

Check (independent recomputation): count the zeros in $1000$ — a power-of-10 number's base-10 logarithm always equals its number of trailing zeros after the leading 1 ($1000$ has 3 zeros), confirming $\log_{10}1000=3$.

Confusing $\log_{10}1000$ (which equals $3$) with $1000$ itself, or forgetting that the common logarithm's base is 10 by convention even when the base isn't written at all.