Imagine a pizza. If a whole pizza was divided into 3 pieces then the size of the pizzas would be pretty big. But if the pizza was divided into 8 pieces the size of the pizzas would be smaller.
Since the numerator is the same you look to the denominator to see which is bigger and smaller. 1/8 is the least because if a pie is cut into 8 pieces and you only get one your piece will be significantly smaller than if it is cut into 3 pieces and you get 1. 1/5 is then in the middle.
