Well, isn't that a happy little number! To round 5.1234 to the nearest thousandth, we look at the digit in the thousandths place, which is 4. Since 4 is less than 5, we keep the thousandths place as it is, making 5.1234 rounded to the nearest thousandth still 5.123.
To round 5.1234 to the nearest thousandths place, we look at the digit in the thousandths place, which is 3. Since 3 is less than 5, we round down. Therefore, 5.1234 rounded to the nearest thousandths is 5.123.
Oh, dude, 5.1234 rounded to the nearest thousandths is 5.123. So, like, you just drop the 4 because it's not big enough to make the 3 round up. Cool, right? Math can be fun... sometimes.