If we were going to summarise all our advice on how to get career capital in three words, we'd say: build useful skills. In other words, gain abilities that are valued in the job market — which makes your work more useful and makes it easier to bargain for the ingredients of a fulfilling job — as well as those that are specifically needed in tackling the world's most pressing problems. So today, we're launching our series on the most useful skills for making a difference — which you can...