[3.5] Ways to learn spells from a different class list

[Curmudgeon]

There's the Extra Spell feat. Debatable if you can get spells off another list (seems to suggest that you can, though).

How would you "suggest" that would work? That is, what's the rule to let you determine the Bard level of a spell like Control Water? For a Wizard or Sorcerer that's a level 6 spell. For a Druid or Cleric that's a level 4 spell. Where's the rule which specifies the Bard level of some spell which isn't on the Bard spell list?

Benefit: You learn one additional spell at any level up to one lower than the highest level of spell you can currently cast.

Or, to put it another way, what's the highest level Druid spell a Bard can currently cast? Because you've got to answer that — with an actual number — before you can determine "one lower than" that; Extra Spell gives you no benefit until you can do that arithmetic.