We may not fully understand Justin Bieber's song “Backpack,” but it sounds like a conversation between an earthling (E) and an extraterrestrial (ET).

E: “Don't try to find your spaceship/It might be out there waiting/ Stay in my backpack forever/ Stay in my backpack forever.

ET: “You know I gotta find my spaceship/ My planet's outside there waiting/ I can't stay in your backpack forever.

We’re not completely sure what’s going on, but one thing is clear: That backpack is too heavy to be healthy. (Steven Spielberg's ET weighed 35 pounds.)

And it’s not the only one. These days, millions of schoolkids are schlepping an overweight backpack.

A Consumer Reports survey in New York City found that sixth graders tote an average of 18.4 pounds every day. But some packs hit as much as 30 pounds.

According to a study published in the International Journal of Environmental Research and Public Health, overfilled backpacks can cause pain in the carrier’s joints, neck, back, spine, and shoulders, as well as fatigue, swelling, and discomfort — not to mention posture problems.

So weigh your child's backpack, and lighten the load if necessary.

The American Academy of Pediatrics says a backpack should be no more than 10% to 15% of a kid's weight. That means, for instance, an 80-pound child should carry no more than 8-12 pounds.

Other things you can do to ease the burden include:

• Use an ultra-lightweight pack with two wide, padded shoulder straps.

• Make sure the straps meet the pack 1-2 inches below the shoulders; the bottom of the pack should never rest more than 4 inches below the waist.

• Use a padded back and waist strap.

• Talk to teachers about making digital assignments so that there are fewer books to carry.