Heard of this before ???

A while back I read that if you have a test and don't know if it's a faint BFP (big fat positive (pregnancy test)) or an Evap line to add a couple of drops of water and if it goes away it was am evap line. And if it stayed it was a line. Has anyone tried this? Is there any one that has a BFP (big fat positive (pregnancy test)) test that can add a couple of drops of water and show us what happenes to find out if it stays when water is added. I have tried it with evap lines many times and they always go away when I add line 4-5 drops of water.

Anyone heard of this ?^