New words：

pupa ['pju:pə] n. 蛹
ladybug ['leidibʌg] n. 瓢虫

Sample Sentences：

One ladybug can eat up to 5, 000 insects in its lifetime.
Within a week or two, the pupa becomes an adult ladybug.
It takes at least eight days for a fly to go from an egg to a pupa.

within [wi'ðin] prep. 在(某段时间、距离或范围)内, 在…里面

New words：

silk [silk] n.丝绸；（蚕）丝；丝织物

Sample Sentences：

This kind of paper was as soft and light as silk.
China is famous for its silk and tea in the world.
It was a stopping point on the famous Silk Road between the East and the West.

New words：

cocoon[kə'ku:n]

n.茧；软罩；保护膜；防护层
v.把…包围起来保护；（用…）完全保护起来

Sample Sentences：

Most worms cocoon in winter.
A cocoon is a kind of silk covering made by an insect.
The children tore the cocoon open to see if there was a pupa.
We were warm and safe, cocooned in our sleeping bags.

silk [silk] n.丝绸；（蚕）丝；丝织物

New words：

certainly ['sə:tənli] adv. 当然；肯定；无疑；确定

Sample Sentences：

They will certainly succeed.
I will certainly see you into the train.
I'm certainly never going there again.
"May I use your motorcycle? " "Certainly".
‘Do you think all this money will change your life?’ ‘Certainly not.’

New words：

certain ['sə:tn] adj. 肯定；确定；确信；无疑
make certain 弄确实；弄清楚；弄明白
for certain 肯定；确定；无疑

Sample Sentences：

We are certain to win the game.
I was certain that he had seen me.
There is no certain cure for this disease.
I can't say for certain when we'll arrive.
Please make certain when the train leaves.