Example 1: Hide on Small, Show on `sm` and Up

Resize your browser window to see the effect. The blue box will be hidden on very small screens (default) and appear on screens `sm` (640px) wide and larger.

I am hidden by default (extra small screens), but visible on 'sm' (640px) screens and up!

Try making your browser window smaller than 640px wide.

This green box is always visible, for reference.