The perfect insert if your super hero requires more absorbency for either nighttime or daytime. Designed to be used with either the Brain Trainer or Waterproof Undies, these inserts add a varying amount of additional protection and absorbency. Size 1: For Brain Trainer Bedwetting Pants and Waterproof Underwear Sizes Small and Medium Size 2: For Brain Trainer Bedwetting Pants and Waterproof Underwear Sizes Large and XL The difference between the Soaker Bombs and the Step-up Inserts are two-fold. 1. Layers - Soaker bombs are 3 layers, folded in half for a whopping 6 layers of microfiber absorbency when folded in half, While Step-up Inserts are 2 layers. 2. Shape - Step-up Inserts have a contoured shape to them, making them lay trimmer and more natural in the underwear. If you want to stack two inserts, the step up inserts will give you a better fit.