100% glycerin USP (VG) is too thick and viscous to be absorbed readily by carto filler material or wicks. So, it must be diluted with something first. People use all sorts of things to dilute Glycerin USP like propylene glycol USP (PG), vodka, Everclear, etc. Sometimes flavorings contain enough PG and/or alcohol to do the dilution. I find that a few drops of distilled water per 1 ml of 100% glycerin USP/nicotine mix works fine for my LR cartos.