If there’s one thing that water slides in open-air pools or lidos have to offer, it’s speed. A competitive water slider explains his three-step technique for picking up speed and getting that nice sliding feeling. First, you need to lie on your back, says Carsten Bücken, chairperson of the German Water Slide Association. But only three parts of the body should touch the slide, so you can pick up speed thanks to the low friction. Here’s how it’s done: Cross your feet over each other so that only one heel touches the surface of the slide. The shoulder blades form the two other points of contact,…