/sass/bootstrap/_component-animations.scss

 1// COMPONENT ANIMATIONS
 2// --------------------
 3
 4.fade {
 5  opacity: 0;
 6  @include transition(opacity .15s linear);
 7  &.in {
 8    opacity: 1;
 9  }
10}
11
12.collapse {
13  position:relative;
14  height: 0;
15  overflow:hidden;
16  @include transition(height .35s ease);
17  &.in {
18    height: auto;
19  }
20}