title: Unity - OnTriggerEnter is not being called
body: <p>I'd like to make an aquarium where diverse fishes are existing.</p>
  
  <p>I tried not to let fishes pass through cubes, but the <code>OnTriggerEnter</code> function doesn't work even if objects enter the trigger.</p>
  
  <p>These pictures represent each of whale and cube inspector factors.
  <a href="https://i.stack.imgur.com/hFrpu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hFrpu.png" alt="whale inspector"></a>
  <a href="https://i.stack.imgur.com/tn88R.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tn88R.png" alt="whale inspector"></a></p>
  
  <p><a href="https://i.stack.imgur.com/myiQ0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/myiQ0.png" alt="cube inspector"></a></p>