Nepalese villagers wave after collecting aid dropped by an Indian helicopter in Khanigaun, Nepal. David Ramos/Getty Images hide caption

itoggle caption David Ramos/Getty Images

This is an artist's imaginative rendering of an atom, but quantum mechanics makes it hard to imagine exactly what an atom is. Richard Graham Farmer/Science Photo Library/Corbis hide caption

itoggle caption Richard Graham Farmer/Science Photo Library/Corbis