Instead of playing outside, children increasingly spend their time watching a TV or computer screen or playing video games.
Even parents' fear of crime has been blamed for limiting kids' opportunity to exercise outside, according to a report from the University of North Carolina, USA.
A recent study from England now suggests that sedentary kids not only risk getting fat, but their grades may suffer, too. The study offered free lessons in a favourite sport to children aged eight to 12.
At the end of the study, the most active kids also scored highest on exams, the BBC News reports. "There is a definite link between those who are active three or four times a week and those youngsters who do better in the classroom," says lead researcher Angela Balding.
Parents who want their children to be more active might call the local fitness clubs. Many now offer classes for kids. The parents can be jogging on a treadmill while the kids practise gymnastics or kickboxing.