#### Answer

The sum of two three-digit numbers, $105$ and $110$, is a three dig number $215$, and not a four-digit number.

#### Work Step by Step

We need to find a counterexample to the statement that the sum of two three-digit numbers is a four-digit number.
One such example would be numbers $105$ and $110$.
$105 + 110 = 215$
$215$ is not a four-digit number.
The sum of two three-digit numbers, $105$ and $110$, is a three dig number $215$, and not a four-digit number.