What about sqrt(-49)?

Can we find a number that goes in this?

(  )^2 = (  )(  ) = -49

Since these are the same number, we'd either have

( - )( - ) = +  or  ( + )( + ) = +

(  )^2 = -49
That's why a square can't be negative!

In fact, this goes for ALL even roots.

4th root of -16     Can't do!

Since no number works in

(  )^4 = -16

There'd be an even number of the guy multiplied together:

(  )(  )(  )(  ) = -16 ... ( - )( - )( - )( - ) = +   or  ( + )( + )( + )( + ) = +
                                                            This negative can't happen!