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To be serious for a moment....
A urine pregnancy test works by detecting a hormone called hCG. Hormone levels are measured in an essentially arbitary unit called IU. In a normal non-pregnant woman this hormone is present at less than 5 mIU/mL in her blood.
A urine test relies on the fact that if left in the bladder for a few hours, the urine absorbs the hormone until it is in equilibrium with the level in the blood.
In a newly pregnant woman, the hormone level climbs slowly in the first couple of weeks after fertilisation, until it's around 15-20 mIU/mL on the presumed due date of the next (and now missed) period. At this point the hormone level rises geometrically, doubling every 24-48 hours.
Most pregnancy tests available to the public will detect the hormone at around 20-25 mIU/mL and these can be used any time AFTER a missed period (ideally around 3 weeks after the presumed exposure to pregnancy).
The best urine to test is that from the first urination in the morning as this has the hormone present in very good equilibrium with the blood.
There are tests available that show a result at levels as low as 10 mIU/mL, however there is a huge downside to these. They will detect before a missed period, but they can give false positives much more often than the higher range tests - this is due to the fact that an estimated 25% of fertilised eggs do not develop into a full pregnancy. In these cases you can get a small hormone spike before the period is due, followed by a largely normal period. In these cases, it is entirely possible that you will see a test tell you you're pregnant, followed by your body telling you quite dramatically that you are not.
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Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
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