Between which two whole numbers is the square root of 18?

4 and 5. The square root of 16 is 4 , the square root of 25 is 5. Those two numbers are the closest to 18.

It is between 4 and 5 because when you set up the question, you put 18 in a square root sign and solve. As In:
                                                     16*Squared  = 4
                                                     18*Squared = It's between 4 and 5
                                                      25*Squared = 5