Learning aims:
1. Learn the new words: match, light, burn, use up, shallow, candleholder, oxygen, one-fifth
2. Learn the important sentences:
① Light the candle.
② As the candle burns, it uses up oxygen in the air.
③ There is about one-fifth oxygen in the air, so the water rises and fills about one-fifth of the jar.
Learning important and difficult points:
分数的表达法：口诀：分子基，分母序，分子>1，分母加s
