Hey, Vsauce. Michael here. Where are your fingers? Seriously. It's a pretty easy question. You should be able to answer it. But how do you know? How does anyone know anything?

You might say, well, I know where my fingers are. I'm looking right at them. Or, I can touch them, I can feel them, they're right here and that's good. Your senses are a great way to learn things. In fact, we have way more than the usual five senses we talk about. For instance, your kinesthetic sense, proprioception. This is what the police evaluate during a field sobriety test. It allows you to tell where your fingers and arms and head and legs in your body is all in relation to each other without having to look or touch other things. We have way more than five senses, we have at least twice as many and then some. But they're not perfect.

There are optical illusions, audio illusions, temperature sensation illusions, even tactile illusions. Can you turn your tongue upside down? If so, perfect. Try this. Run your finger along the outer edge of the tip of your upside down tongue. Your tongue will be able to feel your finger, but in the wrong place. Our brains never needed to develop an understanding of upside down tongue touch. So, when you touch the right side of your tongue when it's flipped over to your left side you perceive a sensation on the opposite side, where your tongue usually is but isn't when it's upside down. It's pretty freaky and cool and a little humbling, because it shows the limits of the accuracy of our senses, the only tools we have to get what's out there in here.

How many memories can you fit inside your head? What is the storage capacity of the human brain? The best we can do is a rough estimate, but given the number of neurons in the brain involved with memory and the number of different connections a single neuron can make Paul Reber at Northwestern University estimated that we can store the digital equivalent of about 2.5 petabytes of information. That's the equivalent of recording a TV channel continuously for 300 years. That's a lot of information. That is a lot of information about skills you can do and facts and people you've met, things in the real world. The world is real, right? How do you know?

We won't be able to ever answer every single question, but walking around in those questions, exploring them, is fun. It feels good. And as always, thanks for watching.