Calculate the product of all positive integers less than $100$ and having exactly three positive divisors. Show that this product is a square.
1. **Identify numbers with exactly three positive divisors:**
   - A number \( n \) has exactly three positive divisors if and only if it is of the form \( p^2 \), where \( p \) is a prime number. This is because the divisors of \( p^2 \) are \( 1, p, \) and \( p^2 \).

2. **List the squares of primes less than 100:**
   - The prime numbers less than 10 are \( 2, 3, 5, \) and \( 7 \).
   - The squares of these primes are:
     \[
     2^2 = 4, \quad 3^2 = 9, \quad 5^2 = 25, \quad 7^2 = 49
     \]

3. **Calculate the product of these squares:**
   \[
   4 \times 9 \times 25 \times 49
   \]

4. **Simplify the product step-by-step:**
   \[
   4 \times 9 = 36
   \]
   \[
   36 \times 25 = 36 \times (20 + 5) = 36 \times 20 + 36 \times 5 = 720 + 180 = 900
   \]
   \[
   900 \times 49 = 900 \times (50 - 1) = 900 \times 50 - 900 \times 1 = 45000 - 900 = 44100
   \]

5. **Verify that the product is a square:**
   - We need to check if \( 44100 \) is a perfect square.
   - Factorize \( 44100 \):
     \[
     44100 = 2^2 \times 3^2 \times 5^2 \times 7^2
     \]
   - Since all the exponents in the prime factorization are even, \( 44100 \) is indeed a perfect square.

\[
\boxed{44100}
\]