When it gets hot outside, it feels so nice to float on your back in a cool pool or lake (once you know how to swim). Well, this is what sea otters do most of the day — what a life! These long, furry, friendly-looking fellows like to float around in a group, called a “raft” because it really does look like a raft. The otters hang onto each other, so they won’t drift apart from each other while sleeping. They even tie themselves to each other with a stringy, seaweed-like plant called kelp. Since sea otters sometimes float in very cold water, they need special thick fur that traps air bubbles and keeps out water so they can float, but will also keep them warm. They have up to 1 million (1,000,000) hairs per square inch! Just to compare, we humans have fewer than 3 thousand hairs per square inch. And that’s why a lot of us still need floaties.
Wee ones: If a raft has 5 sea otters, what numbers do you say to count them?
Little kids: If that raft of 5 otters meets another raft of 2 otters, how many do they have together? Bonus: If another raft joins them, how many does that raft need to make a total of 10 otters?
Big kids: If 10 otters are floating and 1/2 of them have all 4 paws in the air, while the other 1/2 each have 1 paw underwater, how many paws are up? Bonus: If every otter in a 6-otter raft needs to be tied to all the others with 1 piece of kelp, how many pieces of kelp do they need? (Remember not to count doubles between the same 2 otters!)
Wee ones: 1, 2, 3, 4, 5.
Little kids: 7 otters. Bonus: 3 more otters.
Big kids: 35 paws (20 + 15). Bonus: 15 pieces. The 1st otter needs 5 pieces to connect to his friends; the next otter needs only 4, since he’s already tied to the 1st otter. The 3rd otter needs just 3, the 4th needs only 2, and the 5th needs just 1. The last otter is already tied to everyone, so he adds none, giving us 5 + 4 + 3 + 2 + 1.
Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.