Newbie question -how do I add values to a result?

I know that I’m missing something simple here, but the solution is
evading me.

I want to get the values in the fruits field for my first order.

Cart.find(:first, :select => “fruits”)
=>#(Car fruits: “apple, orange, pear”)

Now I want to add grapefruit to that list and save it back into the
fruits field.

How can I do that?

I know it’s something so basic, but I’m missing it.