Definition: Given a statement, generate a question such that the answer is contained in that statement.

Positive Example 1 -
Input: Neurons deliver electrical impulses to other cells
Output: What do neurons deliver to other cells?

Positive Example 2 -
Input: Two lenses to produce magnified images allow for detailed observation of stars.
Output: Two lenses to produce magnified images allow for what?

Negative Example 1 -
Input: Two lenses to produce magnified images allow for detailed observation of stars.
Output: Who is the president?

Negative Example 2 -
Input: Sharks have sharp jaws to catch their prey
Output: What is the biggest shark?

Now complete the following example -
Input: a slide rule is used for subtracting
Output: 


Output: What rule is used for subtracting?