Default value on select tag

When using the Rails helper select_tag, the selected value (ie highlighted value) is a parameter to options_for_select, not select_tag. Eg:

Number of Rows: <%= select_tag(:numrows, options_for_select(%w{10 20 50 100 200 500}, session[:numrows])) %>

cited from: http://soniahamilton.wordpress.com/2009/03/02/ruby-on-rails-select_tag-default-value/