It seems with cigarettes, the more and more time that passes as you smoke, the more and more nicotine you'll be able to handle increases (and how many cigarettes you smoke a day goes up because of it). Basically your tolerance getting higher and higher. But with vaping it seems the longer you vape, the less nicotine you can actually handle and resulting in you dropping your nic level instead of increasing it over time like you do with cigarettes. What's the difference with nicotine in cigarettes and the nicotine in vape juice that causes this?