Sometimes i'm thick sometimes i'm thin, but shape and size comes from within.
Most times i'm hard but sometimes not, i make you run clinching your butt.
While wearing pants and underware, last thing you want is see me there.
I am within you night and day, no matter if you pray or play.
To all the creatures i relate, and im the reason you are late.
Most often i go swim in bowl, with golden cornflakes it i troll,
when flushing once will have its toll.
Sometimes i'm light and sometimes bleak, sometimes i smell and sometimes stink.
I'm seldom found in a sink.
What am I ?