It should be almost undetectable as to how to get this apart without tearing it! Suggestion: Don't do this on a regular basis with your own money -- ...
This is great for including in birthday cards as it is not only a gift but a puzzle as well!
Step 1: Use the newest bill you can find.
Using a $5 or $10 bill is best, because the design on the reverse helps to hide where the crease is, and makes it much harder to figure out how to open it. This will work with any bill, but a newer one will make nice sharp creases.