Explain the sum of the numbers in each row in Pascal's triangle?

the horizontal sums doubles each time the sum of row 1 = 1 row 2= 2 row 3 = 4 row 4 = 8 row 5 = 16 etc etc.......... the horizontal sums doubles each time the sum of row 1 = 1 row 2= 2 row 3 = 4 row 4 = 8 row 5 = 16 etc etc..........