How would you make an animation play on key?

So, I've recently been playing around with an event, where if you press a key such as "a", "b", etc... It'll do something, and I'm wondering how you'd make it play an animation on keydown.

Here's the script.

Script:

function onKeyDown(key)
    if (key~=nil) then
        key = key:lower()
-- What's in here to make it play an animation?
        if (key=="f")  then

            end

    end
end