Steps to reproduce

• Find or make some suspended vines
• Place the gravel block on any block above the bottom block of the vine
• Make sure you place it on the right side!
• This can also work by placing it on top of the vine

The sand/gravel will break the vine below them, but will remain suspended in the air

This is because the sand/gravel will have some vines on its bottom side, which suspends it (although experimenting has revealed that this should not happen)