Change.

The weather?

The hills on the bike?

All those innings of wiffle ball with the kids?

My knees ... my ankles!

Whew!

I have stopped working on the line without standing on those cushy rubber mats that feel like running shoes times ten.

That's what it is ... right?

Maybe I'm getting ...

Naw ... it's the weather ...

Ted, Kooser, "How To Foretell A Change In The Weather"
Rain always follow the cattle

sniffing the air and huddling

in fields with their heads to the lee.

You will know that the weather is changing

when your sheep leave the pasture

too slowly, and your dogs lie about

and look tired; when the cat

turns her back to the fire,

washing her face, and the pigs

wallow in litter; cocks will be crowing

at unusual hours, flapping their wings;

hens will chant; when your ducks

and your geese are too noisy,

and the pigeons are washing themselves;

when the peacocks squall loudly

from the tops of the trees,

when the guinea fowl grates;

when sparrows chip loudly

and fuss in the roadway, and when swallows

fly low, skimming the earth;

when the carrion crow

croaks to himself, and wild fowl

dip and wash, and when moles

throw up hills with great fervor;

when toads creep out in numbers;

when frogs croak; when bats

enter the houses; when birds

begin to seek shelter,

and the robin approaches your house;

when the swan flies at the wind,

and your bees leave the hive;

when ants carry their eggs to and fro,

and flies bite, and the earthworm

is seen on the surface of things.

The author reads this and more of his work here.