The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

A baby has little memory, but as he grows, he remembers things gradually. Later when he comes to .school age, he is guided to improve his memory by practice. Short poems are given to him to study by heart when he reaches primary stage, so that his mind will not get rusty from disuse.
A way to （） a child’s memory.