Sure, there might be some bacteria living on your skin, maybe a flea that might have jumped off a passing dog, perhaps even some lice from a friendly neighbourhood preschool lice plague, but by and large, you should be mostly you.
But it turns out that hardly any of you is actually you.
Surprisingly, a lot of the science in this story comes from nursery rhymes.
You probably remember the one in which little girls are described as being made of "sugar and spice and all things nice". This is totally correct, because they are indeed made of sugars, fats and proteins.
But, would you believe it, another line from the same rhyme is also kind of correct when it says that little boys are made of "slugs and snails and puppy dogs' tails".
Yep, it's true. We humans are mostly made from other life forms.
So here's the really weird part.
Only about 10 per cent of the cells in your body actually belong to you. These add up to about 1–10 trillion cells.
The other 90 per cent of the cells in your body belong to other living creatures. The vast majority of these other living creatures are the 10–100 trillion single-celled beasties (such as bacteria) living in your gut.
In total, these bacteria and their little friends weigh about 1.5kg. The reason that they weigh so little, even though there are so many of them, is that these cells are much smaller than human cells.
The result is that each of us is a strange bacterial-human hybrid. On a cellular level, we are more microbe than man.
The bacteria started colonising your gut as you came down the birth canal, and were pretty well established by the time you were two years old.
Your gut is surprisingly large. If you rolled it out it would be as long as a bus, and if you flattened it out, it has the surface area of a football field.
There are at least 1000 different species of these single-celled critters colonising your gut.
It's quite a fair and reasonable relationship that we have with them.
On one hand, they do their own thing, in the comfort and safety of the human gut. They make little baby copies of themselves, and they communicate with each other.
And when we eat, they eat. They store and redistribute energy, and they maintain and repair themselves.
But in return, they do stuff for us. They make vitamins for us. They also break down carbohydrates that we cannot digest, and extract energy from them.
We humans have about 98 enzymes that can break down carbohydrates. The bacteria in our gut have over 240 enzymes to turn carbohydrates into energy.
In fact, if it wasn't for the single-celled creatures in our gut, we'd all be a lot thinner.
In one study, mice were delivered by Caesarean section in sterile conditions, so they had no bacteria (or their little friends) living in their gut. The mice were then raised in sterile environments, and fed only sterile food.
Compared to their 'regular' germ-laden siblings (who were fed the same food), they ate 29 per cent more and yet, were very skinny, carrying 42 per cent less fat.
And then, when their mice guts were colonised with the single-celled creatures of their 'regular' siblings, they simultaneously ate less and got fatter.
Back in the old days, when food was hard to come by, having bacteria in your gut was an advantage. They would help you extract extra calories from the food. And both you and the bacteria would benefit.
But these days, food is easy to come by. Even so, there are some people who swear blind that they eat hardly anything, and yet put on weight.
If that's true, maybe they just have super-efficient bacteria, wringing those extra calories out of their diet.
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