Just about every family has a stamp collector, and in my family that was my grandfather.
We used to joke about how boring his hobby was, but after he passed away his scrapbook of stamps went to my uncle.
He made a pretty good chunk of change selling the stamps to an antique dealer - thousands of dollars for just a few squares of paper.
Since then, I've taken the value of collectible stamps much more seriously.
If you're interested in collecting stamps, either for fun or profit, then keep your eyes peeled for these 10 rare and valuable American stamps.
1. Hawaiian Missionaries 1851 - up to $750,000
The oldest stamps on this list are also the hardest to track down, and the most valuable.
This early Hawaiian stamp is plain, and even rough, but it was good enough for the missionaries who called the island home in the 19th century.
Just a handful of the stamps have survived through the decades, and less than 20 copies are known to still exist.
If you could get your hands on a pristine copy, it might be worth more than $700,000. But it's more likely any missionary stamps left are worth something like $225,000.
2. Benjamin Franklin 1851 - $120,000
This is about the oldest American stamp you could reasonably expect to find, which makes them worth a small fortune.
The plates that printed them were set by hand, and individual stamps are often unique because of small changes during production. Vintage touches like this also add to the stamp's value.
But you'll have a hard time tracking them down: less than 100 of these stamps (in any condition) are known to exist, so discovering a new one should earn you at least $120,000 - in good condition.
For used or worn out stamps, expect just $10,000 or less.
3. George Washington 1867 - $165,000
Hopefully you recognize the first American president on these classic stamps, but his face isn't what makes them valuable.
There was a limited run of this stamp's 3 cent version in rose color with a B grill - those are the ones that collectors are after.
You don't need to understand the technical details to understand that just 1,000 of these special stamps were ever printed, and only four are known to exist today, so they're in high demand.
Still, if you come across one, somehow, it could be worth $165,000.
4. Abraham Lincoln 1867 - $200,000
This 15 cent stamp features the Great Emancipator, but the draw for stamp collectors is its unusual horizontal grid.
What does that mean to someone who's not obsessed with stamps? Just that it's worth more because that design is rare.
Here's another quirk that makes this stamp valuable: it's one of the oldest that can still legally be used to send a letter.
But you shouldn't, because individual stamps from this set are worth about $200,000 each, especially because of their age.
5. Benjamin Franklin 1 Cent 1867 - $1 million
This stamp, also from 1867 but featuring Benjamin Franklin, uses the same horizontal grid as the Lincoln stamp.
But this stamp is exceptionally rare, and just two are known to still exist, so the odds are stacked against you.
Still, the chance to turn a 1 cent stamp into a cool million dollars keeps us searching.
6. Declaration of Independence 1869 - up to $275,000
Like 1867, 1869 is a key year for U.S. stamp collectors.
This stamp, which features a highly detailed print of the signing of the Declaration of Independence, is another case where small details add to the value.
While the beautiful two-color design is worth a lot, rare variations like stamps with an inverted center are worth even more.
So if you come across a stamp with the founders flipped upside down, it could be worth up to $275.000.
7. Shield, Eagle and Flags 1869 - up to $200,000
This stamps was originally part of a 10-piece set, and uses a groundbreaking two-color printing method.
The design itself is often called "ugly," especially because the two elements overlap in an odd way.
But some samples feature an inverted design, which is of course more valuable than the regular stamp.
Since the printing run was so limited, stamps with the blue flags turned upside-down are highly valued.
The best case scenario for this stamp is an unused 30 cent model with inverted flags in good condition, worth around $200,000.
8. Landing of Columbus 1869 - $275,000
The last piece worth mentioning from 1869 is another two-tone stamp, this time commemorating Columbus' landing in the New World.
The regular stamp actually used inverted printing to match up the two sections (the painting and the frame).
But some samples still came out with inverted images in the center, go figure.
Of course these are the ones to track down, and could be worth $275,000 to you.
9. Benjamin Franklin 1908 - $100,000 in pairs
Yes, it seems like any stamp with the original Postmaster General's image is destined to become a collector's item.
Once again, a special printing error makes these stamps worth much more than face value.
U.S. Postal Service watermarks were applied to these stamps in rows. Because of this, every stamp has the cut-off mark in a different direction.
Whichever way the mark is facing, it makes a stamp unique, so unused Ben Franklin 1908s (found in pairs) are worth about $100,000.
10. Inverted Jenny 1918 - $100,000 (or much, much more)
If you've heard of any famous stamp before, it was probably this one. Truth be told, you probably won't find one of these, but it doesn't hurt to dream!
A printing error left the biplane on this stamp (a JN-4HM, the first plane to deliver mail) flying upside down, which makes them valuable.
But the real profit in finding a Jenny comes from their rarity.
Only 100 of the inverted stamps were printed, and since one is on display in the Smithsonian museum, that leaves just 99 out in the open.
While the stamps are usually said to be worth about $100,000 each, a block of four stamps sold for a whopping $2.97 million in 2014, and just one sold for $1.3 million at auction in 2016.