A recent study shows that dirt may be good for brain and make children smarter because it is a
natural anti-anxiety drug without side-effects.Dr.Dorothy Matthews,an associate professor of biology at The Sage Colleges in Troy,N.Y.,conducted the experiment.She and her colleagues tested how long the mice took to navigate a maze.The result is that the bacteria-exposed mice ran twice as fast as the controls.Their explanation is like this:Serotonin plays a big role in modulating mood,decreasing anxiety.If one is nervous or frightened,he can’t think straight.So serotonin is of significance for learning.In the experiment,the bacteria-exposed mice showed fewer anxiety behaviors than the non-exposed ones.In another experiment,the researchers removed the bacteria from the food provided for the exposed mice and found that those exposed mice were still faster than the controls.Therefore,they came to a conclusion thatthe effect is temporary.(字数：155,实际已经说清楚了.)