As the weather heats up again this weekend, the warning is going out about children inside hot cars.

With the Valley expecting weekend temperatures above 100 degrees Kate Carr, president and CEO of Safe Kids Worldwide, says you need to remember that kids' bodies react differently to heat than adults.

"Their bodies heat up three to five times faster than an adult, making them particularly vulnerable to the hot weather in general and in particular inside a vehicle," Carr said.

The Department of Geosciences at San Francisco State reports that temperatures inside a closed car goes up by about 19 degrees in as little as ten minutes and leaving the window open a crack doesn't make a difference.

According to the San Francisco State Department of Geosciences, 21 children have died of heat stroke because they were left alone in a hot vehicle.

"Heat stroke deaths have been recorded in 11 months of the year, in almost all of the states in the United States," Carr said.

Carr also says they've seen three reports of deaths when outside temperatures were only in the 70s.