It was one of those terribly hot days in Baltimore. Needless to say, it was too hot to do anything outside. But it was also scorching in our apartment. This was 1962, and I would not live in a place with an air conditioner for another ten years. So my brother and I decided to leave the apartment to find someplace indoors. He suggested we could see a movie. It was a brilliant plan.

    Movie theaters were one of the few places you could sit all day and—most important —sit in air conditioning. In those days, you could buy one ticket and sit through two movies. Then, the theater would show the same two movies again. If you wanted to, you could sit through them twice. Most people did not do that, but the manager at our theater. Mr. Bellow did not mind if you did.

    That particular day, my brother and I sat through both movies twice, trying to escape the heat. We bought three bags of popcorn and three sodas each. Then, we sat and watched The Music Man followed by The Man Who Shot Liberty Valance. We'd already seen the second movie once before. It had been at the theater since January, because Mr. Bellow loved anything with John Wayne in it.

    We left the theater around 8, just before the evening shows began. But we returned the next day and saw the same two movies again, twice more. And we did it the next day too. Finally, on the fourth day, the heat wave broke.

    Still, to this day I can sing half the songs in The Music Man and recite half of John Wayne and Jimmy Stewart's dialogue from The Man Who Shot Liberty Valance! Those memories are some of the few I have of the heat wave of 1962. They're really memories of the screen, not memories of my life.

We live in a town with three beaches. There are two parks less than 10 minutes' walk from home where neighbourhood children gather to play. However, what my children want to do after school is pick up a screen — any screen — and stare at it for hours. They are not alone. Today's children spend an average of four and a half hours a day looking at screens, split between watching television and using the Internet.

In the past few years, an increasing number of people and organisations have begun coming up with plans to counter this trend. A couple of years ago, film-maker David Bond realised that his children, then aged five and three, were attached to screens to the point where he was able to say "chocolate" into his three-year-old son's ear without getting a response. He realised that something needed to change, and, being a London media type, appointed himself "marketing director for Nature". He documented his journey as he set about treating nature as a brand to be marketed to young people. The result was Project Wild Thing, a film which charts the birth of the Wild Network a group of organisations with the common goal of getting children out into nature.

"Just five more minutes outdoors can make a difference," David Bond says. "There is a lot of really interesting evidence which seems to be suggesting that if children are inspired up to the age of seven, then being outdoors will be a habit for life." His own children have got into the habit of playing outside now: "We just send them out into the garden and tell them not to come back in for a while."

Summer is upon us. There is an amazing world out there, and it needs our children as much as they need it. Let us get them out and let them play.

"What would the world be if there were no hunger?" It's a question that Professor Crystal would ask her students. They found it hard to answer, she wrote later, because imagining something that isn't part of real life—and learning how to make it real—is a rare skill. It is taught to artists and engineers, but much less often to scientists. Crystal set out to change that, and helped to create a global movement. The result一an approach known as systems thinking—is now seen as essential in meeting global challenges.

Systems thinking is crucial to achieving targets such as zero hunger and better nutrition because it requires considering the way in which food is produced, processed, delivered and consumed, and looking at how those things intersect (交叉) with human health, the environment, economics and society. According to systems thinking, changing the food system—or any other network- requires three things to happen. First, researchers need to identify all the players in that system, second, they must work out how they relate to each other, and third, they need to understand and quantify the impact of those relationships on each other and on those outside the system.

Take nutrition. In the latest UN report on global food security, the number of undernourished (营养不良) people in the world has been rising, despite great advances in nutrition science. Tracking of 150 biochemicals in food has been important in revealing the relationships between calories, sugar, fat and the occurrence of common diseases. But using machine learning and artificial intelligence, some scientists propose that human diets consist of at least 26,000 biochemicals—and that the vast majority are not known. This shows that we have some way to travel before achieving the first objective of systems t hinking - which,in this example, is to identify more constituent parts of the nutrition system.

A systems approach to creating change is also built on the assumption that everyone in the system has equal power. But as some researchers find, the food system is not an equal one. A good way to redress (修正) such power imbalance is for more universities to do what Crystal did and teach students how to think using a systems approach.

More researchers, policymakers and representatives from the food industry must learn to look beyond their direct lines of responsibility and adopt a systems approach. Crystal knew that visions alone don't produce results, but concluded that "we'll never produce results that we can't envision".