Why are only some batteries rechargeable?

This week, where Vitamin D comes from, if lab-grown protein is vegan-friendly, how mammals evolved to dive deep underwater, how genes can be used to track evolution, how chillies create a burning sensation in the mouth, whether feet change shape as we age, what makes a battery rechargeable, and why tropical seas tend to be blue and clear. Dr Chris Smith and Lester Kiewit have the answers...