Who's invoking yeild?

This is a question that, I think,. straddles both Ruby and RoR. I'm
gonna post it here.

In some sample code, I see the following in the <head> section:

    <%= yield(:head) %>

I see that the yield is returning something (i.e. ":head") ... but why?
And to whom? How can I tell?

<%= yield(:head) %> (presumably in one of your layouts) yields to any view doing something like

<% content_for :head do -%>

This text will be in the location of the html where <%=yield(:head)%> is

<% end %>

This is useful for doing things like include genormous javascript libraries like tinymce only when needed in views that are editing with wysiwyg text boxes.