Cat brings presents

by Claire
(Cheshire)

Our beautiful female cat Saphire age 4 years, frequently brings us presents, my children and i will be siting downstairs and we will hear saphire sqeaking quite loudly and down she will come with a pair of the childrens knickers in her mouth (clean ones of course, the girls keep their underwear in baskets under the bed) she will place them gently on the floor and then walk out. we always say thankyou to her as she goes to so much effort.
We end up with at least 7-8 pairs a day! i have tried leaving them on the floor but she just brings more. On the odd occasion it will be something different like tonight we got a toothpaste lid but 9 times out of ten it's knickers. why does she do this??