Why doesn't a snail's shell get too small as it grows?
蜗牛长大了，壳怎么不会太小？
蜗牛背上的壳，看起来像一间小房子。
The shell on a snail's back looks like a little house.
可是蜗牛长大了，这间“房子”怎么不会太小？
But when the snail grows, why doesn't this “house” become too small?
它不用到处找一个更大的壳。
It doesn't have to go around looking for a bigger shell.
壳是它自己做出来的，不是从外面捡来的。
It makes its own shell rather than picking one up from outside.
蜗牛的身体上，有一层紧贴着壳的组织。
The snail has a layer of tissue on its body that lies against the shell.
这层组织会做出长壳用的材料。
This tissue produces the materials used to grow the shell.
新壳从哪儿长出来呢？
Where does the new shell grow?
就在蜗牛把头伸出来的那个开口边上。
Right around the edge of the opening where the snail sticks its head out.
身体长大时，那里会一点点加上新的壳。
As the body grows, new shell is added there little by little.
所以，壳变大不是把旧壳拉大，而是在开口边上接着长。
So the shell gets bigger by continuing to grow around the opening, not by stretching the old shell.
这间“小房子”，原来是蜗牛自己一点点长出来的。
It turns out that the snail grows this “little house” itself, bit by bit.
