Se deals with the sensory world the way Ne deals with ideas. Like Ne, which is constantly ruffling through possibilities and ideas in a random-like manner, Se is floating through the sensory world randomly. Both lack a focus until something interests them, at which point the object is siezed upon and looked at in closer detail.
For example, me, my sister and my brother would play this game on long car trips - all of us looking out the window to see what animals we could spot. Common ones like sheep and cows got 1 or 2 points each, and uncommon ones (say you spotted a giraffe wandering down the street ) would be worth more. Me and my brother (INFP) were usually pretty even, but my sister (ESFP) would be scoring (i.e. having to concentrate on a piece of paper and add up the scores we were calling out) as well as play the game, and she'd beat the pants off us, every single time. It's Se's master domain.
Si is pretty much a mystery to me. Apparently I'll learn how to use it one day.