What does this "head" mean? The top part of lettuce or a counting unit of a whole lettuce?
ex)Imagine you are watching a science fiction movie. In a scary scene, an alien hatches from an egg. When it does, the sound is very realistic! How is this possible?
For this scene, a person tears a head of lettuce in half to make this sound...
Context is always important; labelling is rarely important.
- source7
b : the foliaged part of a plant especially when consisting of a compact mass of leaves or close fructification <a head of lettuce>