Five-year-old David explained about his nine-month-old sister: ''She doesn't talk yet - she just hums.'' Can you quote a child who has said something wise, amusing, or just wonderfully childlike? Please share it in the new Home Forum Competition. Thanks for all your literary exertions in previous competitions. Here is a chance simply to recall a few actual words - certainly no more than 25 - spoken by someone of 10 years or younger whom you personally know. Only one example per entrant, please. Those received by April 5 will be considered for certificates to be announced April 30. Sorry, no entries can be returned. Address: The Home Forum Competition, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115.
Oh, and who's David? He's a grandchild of Elsie U. Huber of Long Island. She sent his remark and others with fond memories of the Monitor's ''Small Fry'' column. Then Ruth Barrick Kiesel of Kansas City wrote us on the same subject. So , in response to popular demand - the what-children-say competition!