What was it about snails that made the writer collect them for his friend on that day in particular?

还原为陈述句，可能是：

1. The idea  about snails made the writer collect them for his friend on that day in particular.

变成强调句：

2. It was the idea  about snails that made the writer collect them for his friend on that day in particular.

就划线部分提问：

3. What was it about snails that made the writer collect them for his friend on that day in particular?

强调句的特殊疑问句式举例：

疑问词+ is (was) it + that +句子的其他成分，特殊疑问词即是被强调的成分。例如：

Who was it that found your pen in the classroom yesterday?

When was it that the Second World War broke out?

Where was it that you lost your cell phone?

How was it that he managed to get my address?

How long ago was it that you last saw her?

What was it that had made you so sad?　　

Why was it that silver was not widely used as conductor?　　

Which book is it that you want?　　

Whose umbrella was it that you took away?