Pebbles and Primes

Hand out stones and arrange them in rows to discover primes and multiplication.

• 6 stones: Arrange in rows. Two rows of 3 (or three rows of 2) — 6 = 2×3, so 6 is not prime.
• 7 stones: Only one full row of 7 (or 7 rows of 1). No other equal rows — 7 is prime.
• 8 stones: Two rows of 4 (or four rows of 2) — 8 = 2×4, so 8 is not prime.

Guess the next prime: 11 — one row of 11 (or 11 rows of 1); no other equal grouping, so 11 is prime.

Through this pebble activity you learn both what primes are and how multiplication relates to rectangular arrangements.