'Young people just don't have the opportunity to get enough exercise these days and this is beginning to show in the increase in weight-related problems among this age group.' - What might be the writer's motive in making this claim?

Basically, 'bias' expresses the difference between fact and opinion. But texts are not always so simple - writers can make a highly-opinionated piece appear factual, or can choose to present facts and statistics in a selective, biased manner. Learning to spot bias will make you a careful, critical reader.