How do I make lots of classes aware of each other?

I'm apparently missing something fundamental in my knowledge of classes
and how they interoperate.

Let's say I have lots of classes, such as the following:

class Foo
  def method
    puts "hi"
  end
end

class Bar
  def method
    puts "hi"
  end
end

And I want to write another class that uses all of those classes.  For
example:

class Baz
  foo.method
  bar.method
end

What I'm doing right now is initiating all the classes as follows:

$foo = Foo.new
$bar = Bar.new

So I then have:

class Baz
  $foo.method
  $bar.method
end

But this has to be wrong (?)

I could make them all modules and then include them as needed, but I'd
still end up with:

class baz
  include Foo
  include Bar
  ...x20ish

Essentially, I have about 20 classes and many of them talk to each
other, so they have to be aware of each other.  What's the right way to
structure all of this?

Many thanks in advance!

Andrew

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