Mark's class planted fruits and vegetables in the school's garden. Mark liked watering them and he especially liked watching the plants grow best. One morning, Mark stopped to check the garden. He saw something surprising. He hurried into his classroom. "Mr. James!" he shouted to his teacher worriedly, "The cabbage is covered with a kind of insect!I think they're eating it!Please do something to save our plants!"Mr. James just smiled. He showed Mark a small box, Mark. I think I know what caused the problem. There's something in this box that might help. "
Mr. James led his class out to the garden. "Insects called aphids are eating our cabbage, " he said. "We can use what we know about food chains（链） to work out the problem. "Mr. James said and then carefully opened the small box, "How can they help? "
Mr. James smiled again. He pointed to the cabbage and said, "Cabbage is a producer, and aphids are customers that are eating the cabbage. Ladybugs are customers, too. But they like to eat aphids. Let's just wait and see what will happen over the next few days. "
For the next week, Mark watched as the aphids died off day by day. One morning he came to the garden to study the cabbage. He saw that it looked healthy and strong. There were no more aphids on it, either!What Mr. James did really worked!
