elasticsearch prefix query for multiple words to solve the autocomplete use case

How do I get elastic search to work to solve a simple autocomplete use case that has multiple words?

Lets say I have a document with the following title - Elastic search is a great search tool built on top of lucene.

So if I use the prefix query and construct it with the form -

{
    "prefix" : { "title" : "Elas" }
}

It will return that document in the result set.

However if I do a prefix search for

{
    "prefix" : { "title" : "Elastic sea" }
}

I get no results.

What sort of query do I need to construct so as to present to the user that result for a simple autocomplete use case.