Alright, then let me try this one...
When you tap Reflecting Pool, you choose what mana to add on resolution (you don't choose what type you're adding as you play it, though functionally (since it can't be responded to) there isn't a difference).
Reflecting Pool asks this: "If you were to use any other land to produce mana right now, what could it possibly produce under the current game state if you could pay the cost?" River of Tears will only be able to produce
, not '
' (note the quotations), depending on the game state.
Does that make sense?