```ruby
# frozen_string_literal: true
module UsersHelper
  # Returns the Gravatar for the given user.
  def gravatar_for(user)
    gravatar_id = Digest::MD5.hexdigest(user.email.downcase)
    gravatar_url = "https://secure.gravatar.com/avatar/#{gravatar_id}"
    image_tag(gravatar_url, alt: user.name, class: 'gravatar')
  end
end
```

Use "mystery man" if there's no Gravatar image
user
```ruby
# frozen_string_literal: true
module UsersHelper
  # Returns the Gravatar for the given user.
  def gravatar_for(user)
    gravatar_id = Digest::MD5.hexdigest(user.email.downcase)
    gravatar_url = "https://secure.gravatar.com/avatar/#{gravatar_id}?d=mm"
    image_tag(gravatar_url, alt: user.name, class: 'gravatar')
  end
end
```
assistant