Playing with grandchildren helps older people keep their wits about them - just don't do it too often!
That's the conclusion of a study that found grandparents who looked after young children once a week were mentally sharper.
However, playing with kids five days a week has the reverse effect!
The Australian study looked at 186 women, aged 57 to 68, who took three different tests of mental sharpness.
The volunteers were also quizzed on how demanding their own children had been during the past year.
Among the 120 grandmothers, those who spent one day a week taking care of grandchildren performed best on two in three tests.
But those taking care of grandchildren for five or more days a week did significantly worse on one of the tests, which assessed working memory and mental processing speed.
The volunteers were from the Women's Healthy Ageing Project study in Australia and the findings are published in Menopause, the journal of the The North American Menopause Society (NAMS).
Dr Margery Gass, executive director of NAMS, said: "We know that older women who are socially engaged have better cognitive function and a lower risk of developing dementia later, but too much of a good thing just might be bad."
The researchers admitted they were surprised by the finding that looking after grandchildren five or more days a week could be detrimental for mental health.
The report said: "The more time grandmothers spent taking care of grandchildren, the more they felt that their children had been more demanding of them, so maybe mood plays a role in the unexpected result."
Dr Gass added: "Because grandmothering is such an important and common social role for post-menopausal women, we need to know more about its effects on their future health. This study is a good start."