I'm trying to read lines from a file that might contain something like:

Here is a line with a variable to interpolate: #{var}

Now when I read it in like:

  var = 99
  File.foreach("junk.rbt") {|line|
    puts line
  }

of course I get the same line in the file with the #{var} in it.
I'd like to get:
Here is a line with a variable to interpolate: 99

I suspect there must be a way to do this since using "" causes 
interpolation, how can I get an existing String to interpolate itself?

Phil