I am light but strong,
Sometimes I am long
I can write but cant read
To humans I don't belong
Flight is guaranteedI will help you sleep
Even more deep
Or in your hat
I might not be cheap
You could find me in a cat
What am I?
The answer is
a feather.
I am light but strong,
Feathers are lightweight, but sturdy enough to support the weight of large flying birds.
Sometimes I am long
Some feathers (e.g., the tail feathers of peacocks, parrots and parakeets) are quite long.
I can write but can't read
Feathers can be made into quill pens, which are writing implements, but they have no ability to read.
To humans I don't belong
Feathers belong to birds, not to humans.
Flight is guaranteed
Presumably this refers to the fact that feathers contribute to birds' ability to fly, but "guaranteed" seems inappropriate, since some birds (that have feathers) cannot fly.
I will help you sleep
Even more deep
Feathers are used in pillows.
Or in your hat
Feathers are used as decorations in hats.
I might not be cheap
Some feathers (especially long ones; see above) may be expensive.
You could find me in a cat
Cats eat birds. While I imagine that they try to avoid eating the feathers, they probably cannot avoid doing so occasionally.
I think you are:
Helium
I am light but strong
Helium is lighter than air, and in large quantities can be strong enough to lift heavy objects
Sometimes I am long
Helium is used in balloons, twisting balloons can be long and thin
I can write but cant read
Twisting balloons can create letters?
To humans I don't belong
Helium has been around around forever, so it doesn't belong to us
Flight is guarenteed
Helium Balloons can float
I will help you sleep, Even more deep
Helium has been used in anaesthesia, which is a deeper form of sleep
Or in your hat
Helium is in the air, so it could be in your hat
I might not be cheap
Helium has been being used a lot and we could have a shortage, so the value is always rising
You could find me in a cat
Air contains a small amount of helium, so it can be within the cat's lungs
Some other interesting one that would also work:
Aluminium
I am light but strong,
Aluminium is indeed light and strong
Sometimes I am long
you can make long things with it (wires, poles, &c)
I can write but cant read
You can make pens with aluminium
To humans I don't belong
It is something that will poison humans, so why would it be in humans?
Flight is guarenteed
Before composites, many airplanes were made with aluminium.
I will help you sleep Even more deep
Same with the humans part. You could die from aluminium poisoning.
Or in your hat
Helmets
I might not be cheap
With alloys, it could be expensive.
You could find me in a cat
If the cat was being stupid.