Find the product of the greatest common divisor and the least common multiple of $18$ and $42.$
First we find the prime factorization of each number: $$18=2\times 9=2\times 3\times 3=2\times 3^2$$ and $$42=2\times 21=2\times 3\times 7.$$ The common factors are $2$ and $3,$ so $\gcd(18,42) = 2\times 3=6.$

The least common multiple is formed by multiplying together the highest powers of all primes occurring in the factorization of either $18$ or $42:$ $$\text{lcm}(18,42) = 2\times 3^2\times 7 = 2\times 9\times 7 = 2\times 63 = 126.$$ Thus, the product of the $\gcd$ and the $\text{lcm}$ is $6\times 126=\boxed{756}.$

(You can check that this product is equal to the product of the original two numbers, $18$ and $42$. Is that just a coincidence?)