Note: This must be done in your head only.
Do NOT use paper and pencil or a calculator.

Take 1000 and add 40 to it.. Now add another 1000. Now add 30.
Add another 1000. Now add 20. Now add another 1000
Now add 10. What is the total?

Now, be truthful

And another to see if you really are sharp today:

How much dirt in a hole 10 feet long, 3 feet wide and 8 feet deep?