Here's why:
* Metaphor: A metaphor directly compares two unlike things without using words like "like" or "as." In this case, "heaven" is being compared to a drain.
* Simile: A simile uses "like" or "as" to compare two unlike things.
The sentence implies that heaven is somehow similar to a drain, even though those are very different concepts. It's a figurative way of saying something about heaven, likely that something valuable has been lost or wasted.