Colider Issue: a rigidbody colider drops through the box colider

HI There,
thank you in advance for your viewing.
Here is a very simple but confusing issue here.
I am practicing with the tutorial on 1

However, as the attached images mentioned, I notice that the cube (rigidbody colider) drops through the earth (box colider).
Would you plz kindly lemme know why that occurred. And I will highly appreciate it if you could help me to fix.

Do either have the isTrigger checkbox checked in the editor? A rigidbody with a isTrigger collider will fall through another (aka: no longer solid). In this case, I fixed a previous problem by adding another collider with isTrigger unchecked.