Hi, here's your problem today. This problem was recently asked by Apple:

You are given an array. Each element represents the price of a stock on that particular day. Calculate and return the maximum profit you can make from buying and selling that stock only once.

For example:

```
[9, 11, 8, 5, 7, 10]
```

Here, the optimal trade is to buy when the price is 5, and sell when it is 10, so the return value should be 5 (profit = 10 - 5 = 5).

Here's your starting point:

```
def buy_and_sell(arr):
#Fill this in.
print buy_and_sell([9, 11, 8, 5, 7, 10])
# 5
```