Below, Down, Under, Beneath
There's a toe below ground.
So, where is the toe now? It's below ground. Imagine yourself digging it out, and thinking... ugh, why does it have to be below ground like this? So irritating. Will I be able to sew it back on in time to save it?
か, げ
した, さ, くだ, お, しも, もと
None
You keep digging for the toe below the ground, and instead of hitting a toe you hit something hard and metallic. You keep digging and digging around it, but it's huge. Turns out, you found yourself a car (か). Keep digging until you unearth the car. Inside, you find the toe you were looking for.
There's another useful on'yomi reading that just isn't useful enough for you to learn now. That reading is げ, but we're only going to worry about that much, much later when you have to learn one or two vocab words that read 下 as げ. That's the problem with really common kanji - too many readings.