Scientists have come a step closer to gaining complete control over a mind, even if that mind belongs to a creature the size of a grain of sand. A team at Harvard University has built a computerized system to manipulate worms — making them start and stop, giving them the sensation of being touched, and even prompting them to lay eggs — by stimulating their neurons individually with laser light, all while the worms swim freely in a petri dish. The technology may help neuroscientists for the first time gain a complete understanding of the workings of an animal’s nervous system.