Anyone accident-prone might want to take more care than usual - experts have discovered that sustaining a mild injury could lead to brain damage.
Even a slight bang on the head - for instance after falling off a bicycle - can cause brain damage affecting thinking and memory, research has shown.
Scientists compared 53 people who had suffered mild or moderate head injuries several days earlier with 33 uninjured individuals.
Mental ability tests showed that thinking and memory skills were 25% lower in the first group. Scans showed this was related to disruption of nerve fibres in the brain.
A year later test scores were the same for people with and without head injuries, but areas of brain damage were still seen in participants who had suffered blows to the head.
Study leader Dr Andrew Blamire, from the University of Newcastle, said: "We studied patients who had suffered clinically mild injuries often from common accidents such as falling from a bicycle, or slow speed car accidents.
"This finding is especially important, as 90% of all traumatic brain injuries are mild to moderate."
The findings are published in the online issue of the journal Neurology.
Dr Blamire said the results showed that thinking skills recovered over time as the brain rewired itself.
"The areas of brain damage were not as widespread across the brain as previously, but focused in certain areas of the brain, which could indicate that the brain was compensating for the injuries," he added.