Researchers at Dell Children's Medical Center in Austin, Texas took temperature readings inside cars every single month of the year.
Even on cooler days as low as 68 degrees temperatures inside the cars reached 105.
Experts say children can overheat much more quickly than adults because of their smaller size.
In other health news, kids who are strong readers in early elementary school may do better on I.Q. tests in high school.
Researchers in the UK examined reading and intelligence tests taken by sets of twins throughout their school years.
Those who had good reading skills by age 7 later did better on vocabulary and tests of verbal fluency and reasoning.
Experts say this suggests kids who are not doing well with early literacy may need extra intervention.
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