https://gist.githubusercontent.com/chriseppstein/1215856/raw/2f359e67c6b5a8e61512373f5b9f26842ad7623d/6_media_queries.scss

@mixin respond-to($media) {
  @if $media == handhelds {
    @media only screen and (max-width: 479px) { @content; }
  }
  @else if $media == wide-handhelds {
    @media only screen and (min-width: 480px) and (max-width: 767px) { @content; }
  }
  @else if $media == tablets {
    @media only screen and (min-width: 768px) and (max-width: 959px) { @content; }
  }
}

#sidebar {
  float: left;
  width: 300px;
  @include respond-to(handhelds) { float: none; }
  @include respond-to(wide-handhelds) { float: none; }
  @include respond-to(tablets) { width: 240px; }
}