Kayak bottles

I want a bottle that will stay in place under the shock cords on a kayak's foredeck.

That most likely means one concave side to align with ridge running the length of the deck.

(And I want them both for cold and hot drinks.)

Bike helmet, safe for jungle gym use.

Sadly, some times kids are suffocated by their bike helmets while playing in jungle gyms. They shouldn't wear their helmets while climbing, but they will.

I can think of two approaches to making the helmets safer for climbing.

The first is shape. The helmets worn by trawler crews look like they are less likely to catch anything.



The second is the strap. Can someone please design a strap that will keep the helmet in place when jerked, yet releases when pulled by the kid's body weight for a couple of seconds?

Vertical monofin

Yes, I know human anatomy, like seals' and whales', is best suited to a monofin with horizontal flukes.

Still, trying to swim like a fish, with a vertical monofin/tail, would be an interesting workout.

The product is not totally utopian, I did find one on Youtube. But I don't know where to get my hands...uh, feet, in one.

First the fundamentals: A material with negative dimensions

It's obvious: A wall with negative thickness gives a house with Potteresque properties: Small on the outside, but big inside.