Scratch

You could use two options. One is, when your character is touching wall (you might want to make it a certain colour) then: if touching colour [] (the wall's colour) turn 180 degrees, move 20 steps. Another option is sensors, they are invisible different coloured lines that go in a square around your character. Now, i've never used them but from studying other peoples stuff, when the colour of the sensor is touching a wall or a floor, it sets a variable, usually xcollision for floors, ycollision for walls, to 1. When it is 1 it means that the sprite will stop moving down. This will only work if the sprite can move when x or ycollision is 0. You might want to go to the advanced topics and post a new topic on sensors. Hope this helps!