Expectation value and variance of the sum of random variables with conditions

Let $ X_1,X_2,\dots,X_n$ be independent and identically distributed Poisson random variables and define the partial sums $ S_n=\sum\limits_{i=1}^n X_i$ .

I would like to calculate the expectation value $ \mathbb{E}(S_n)$ and variance $ \text{Var}(S_n)$ .