combinatorics - Combinatorially prove that $sum_{i=0}^n {n choose i} 2^i = 3^n$

So I'm not sure at all how to prove things using a combinatorial proof. Where to do I start? What do i need to think about etc. For example how would i prove

$$\sum_{i=0}^n {n \choose i} 2^i = 3^n$$