It sways from side to side on its pointy feet, considering this question in depth.

{you are friend. i am spirit. tree is tree. animal is animal. monster is monster. friend does friend things, spirit does spirit things, animal does animal things, monster does monster things. friend things are many. spirit things mostly talking. animal things mostly moving around, sometimes eating grass. monster things mostly trying to hurt friends and spirits and animals.}

"So I'm not a monster? Even though I'm big and invisible and have tentacles?" 

{???} says the spirit, somehow. {you are friend! big invisible tentacle friend!!} It waves its tiny pointy arms for emphasis.

"Wilbur this thing is so cute," she whispers to her brother. 

"Yes. Yes, it is," he whispers back. 

"What kinds of monsters are there? Why do they try to hurt stuff?" 

{many kinds! monster is monster, does monster things because of being monster.}

It sways thoughtfully again, then looks around a little, and finally points in a direction.

{small monster there! green slime. is reactive monster. hurts animals always, but only hurts friends who touch it or hurt it first.}

In the direction it pointed, there is a round blob of translucent green jelly, hopping along the forest floor. The blob is about a foot in diameter, when not distended by the force of its motion.

Lucy immediately goes up and pokes it. 

It wobbles in place for a moment, then launches itself directly toward her. It stings when it hits, and the impact has enough force to slightly stagger an ordinary human; Lucy, of course, is rather harder to stagger than that. It's still going to bruise a little, though.

"Are monsters people?" she asks the spirit. 

{not mostly. special monster maybe. special monster is like spirit, is only one of each kind.}

The green slime bounces off Lucy, lands, and hops aggressively into her again.

She plucks it out of the air and eats it.

It tastes like slightly grass-flavoured water, and has the consistency of particularly bouncy jelly.

Also, although it didn't look like it had anything inside it, there turns out to be a small rubbery ball somewhere amid the goo. It tastes sort of unappetizing, not even as foodlike as grass.

{small thing is gel from slime!} the spirit says helpfully. {useful. not good to eat.}

She spits out the gel. "What's it good for?" she asks, examining it.

{can make thing!}

And as soon as the spirit says that, she can start to... 'read', maybe, in the same way she 'hears' meaning in the spirit's speech... a list of things one could make with the gel. Unfortunately, all the entries are mostly blurred out, with just the {1/? gel} part visible.

"Whoa what the heck is this."

{can make thing!} the spirit repeats. {need more things though. can get wood from tree, make torch!}

"Why are there fuzzy recipes in my brain." 

"Why are there what?" 

She hands him the gel. 

"..." 

{is how make thing,} the spirit explains.

"Why is how make thing in my brain???"

{??}

It looks up at her in confusion.

"Why does the gel tell me how to make things with it?"