Hey FoxxTrott! I didn't code this game, so Bio_Hazard1282_rPi3 will probably have a better response. But I think I can explain it. What the loop is doing is checking if there is a pixel at (x,y). If there is, you die. If there isn't though, the loop continues and draws a pixel, representing you, at (x,y). Think of the x and y coordinates as being future x and future y positions. If they are the future positions, then it makes sense to check if the player will run into a tree and die at that point in the code. If there is nothing there though, it will just continue as normal. If you're actually wondering about the purpose of "Pxl-On(31,X", it just displays a pixel where the player is. And to be clear, the pxl-test command makes sure that you don't run into anything. If you are still confused, Bio_Hazard will probably post an informative reply (he's the one who wrote the code). If you don't understand what the pixel commands do in general, just look them up in this wiki. This wiki contains an index of most (if not all) available TI-Basic commands.