Parents are often clueless when their kids start smoking e-cigarettes, a new study finds.

On the other hand, Mom and Dad usually can tell if their children take up traditional smoking, said researchers from the University of California, San Francisco.

Having strict household rules against any form of tobacco is the best form of prevention, researchers found. And those rules should apply to adults, too.

“Tobacco use by children is troubling, and dentists, like all health care providers, should be concerned about preventing youth tobacco use,” study co-author Dr. Benjamin Chaffee, an associate professor in the university’s School of Dentistry.

“We know that tobacco-free homes are a key tool to help prevent smoking by kids,” he said in a university news release.

The study included parents of more than 23,000 kids aged 12 to 17. Researchers found the parents were less likely to know or suspect that their child used