Teacher:In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 
Teacher: Now, understand the problem? Solve this instance: Dissolution of limestone forms caves. Rain gets carbon dioxide from the air. When rain goes into soil it creates an acid. The acid dissolves the limestone along cracks. Some will dissolve enough to form caves. 
Student:
How do caves form?