English Is Weird

We were filling the new raised beds - 2'x6'x2' ribbed metal ovals - with soil.  The soil - eight heavy 3cu foot bags - had to be moved to the beds from their resting place, outside the garden fence.  This took coordination and planning.  I left it to them to figure it out.  

Some bags were rolled.  One was tipped over the fence. Most were carried by a gaggle of garden kids of all sizes.  It took two of the four recess sections, but we managed to fill the beds, keeping most of the soil inside.

Fourth and second graders joined the party, which had moved from breaking up clumps of soil to evening it out with hand rakes and small trowels.  There were giggles and questions and then there was silence.

A second grader's face was covered with soil.  

A fourth grader's face was abashed.

Sorrowfully, the big girl said Not by purpose, I got her face dirty.

It took a minute or two to wipe the little one off.  All the while I was playing that sentence over in my head.  Not on purpose, but by purpose.  

English is not her native tongue; she's conversational if grammatically inconsistent.  Idioms are hard. Was she conflating by accident and on purpose?  

And why don't we say by purpose and on accident? 

Are we next to - by - an accident?  Can an accident have intentionality and cause itself?

I understand being on task, but being on purpose? 

Who decided which one went where anyway?

Interrchangeable idioms with interchangeable prepositions have been dogging me all day.  I hope you can escape the same fate.