How is crystal meth made from pseudoephedrine allergy pills...???

[Curiously Asking 2008]

I am confused here...I have hay fever allergies quite a bit, so I usually take over-the-counter pseudoephedrine allergy pills practically 3 or 4 times a day, and these pills make me SO sleepy. This is why I am confused about how crystal meth is manufactured from these pills, when crystal meth is a well known stimulant, and these pills make me want to take a nap every time I take them. So, exactly how is it manufactured from these pills, and in the manufacturing of the crystal meth, what is the chemistry breakdown change that causes it to become a stimulant, instead of a depressant? Any and all feedback would be greatly appreciated. Thank you!