Thread.join blocks the main thread

Calling Thread.join blocks the current (main) thread. However not calling join results in all spawned threads to be killed when the main thread exits. How does one spawn persistent children threads in Ruby without blocking the main thread?

Here's a typical use of join.

for i in 1..100 do
  puts "Creating thread #{i}"
  t = Thread.new(i) do |j|
    sleep 1
    puts "Thread #{j} done"
  end
  t.join
end
puts "#{Thread.list.size} threads"

This gives

     Creating thread 1  
     Thread 1 done  
     Creating thread 2  
     Thread 2 done  
     ...  
     1 threads  

but I'm looking for how to get this

    Creating thread 1  
    Creating thread 2  
    ...  
    101 threads  
    Thread 1 done  
    Thread 2 done  
    ...  

The code gives the same output in both Ruby 1.8.7 and 1.9.2