How do magnets multiply? What keeps an aeroplane in the air? How do wild animals avoid incest? It's open season on science questions in this week's Naked Scientists. We'll find out if oil extraction leaves a cavity, can cranberry juice cut urine infection rates and what happens when two lightning bolts collide? In the news, evidence of bipedalism in an early human ancestor, how oily fish helps avoid common causes of blindness and how smartphones are taking the pain out of cardiac rehabilitation. Plus, in Kitchen Science, the unexpected physics of a flying balloon.