It seems straight forward, but some students can still struggle with the fact that 688,999 is less than 700,000. Because 8s and 9s are larger numbers than 0s, right?
And even among those who do know that 700,000 is greater, it's another skill to know what it would take to make it less. So what would need to happen to 688,999 to make that so?
Teachers and parents can also manipulate index cards with numbers in a "home number talk." So if a student has three cards with a 2, a 7, and a 4, what is the largest number he can make with that? The smallest? What if the 2 is traded for a 5? And so on....
The Standards of Mathematical Practice (see NCTM site) must be embedded in daily mathematics instruction. STEM teachers know this.