Imagine I walk up to you and hand you a sealed cardboard box and asked you how long it would take to sort everything inside.

What's in the box?

Not sure. Toys?

Okay, how many 'maybe toys' are in the box?

No idea. Maybe somewhere between 5 and 50.

Sure... well, how do you want them sorted? By color? Size? Type?

Hah, you're the sorting expert, you tell me. Whatever is appropriate as long as I can tell stakeholders they're sorted according to best practices. So, how long is it going to take? I also have four other boxes that need sorted, and I need all of them done in the next 5 minutes, so if you could give me an estimate that I could take back to the stakeholders...

Are the other boxes similar to this one?

I think so? Again, you're the sorting expert here, I'm just the messenger.

you open and peek into the box Okay, well it looks like there are only a few items in here. I can probably get this box sorted in 30 seconds or so, but let's double it to a minute to be safe and account for any variability in the other boxes. That should be right around 5 minutes.

Awesome! See you in 5!

You begin sorting, all goes well until you reach the bottom of the box - where you find hundreds of different colored, different sized marbles.

The timeline is ruined, the stakeholders will be disappointed. Everyone continues to question why you, a professional sorter, can't just give accurate estimations.