I once explained it to my son like this: "Go and take 20 of your Pokémon cards and we assume they are all worth the same and I get 20 Euro. Now for how much of my 20 Euro would you sell me one of your 20 cards?" He said at least 1 Euro. "Now if I go and get another 20 Euro. How many of my 40 euro would you line to have for one of your cards?" He though about it and said "At least 2". My son is ten years old. He gets it.