I'll take a stab at explaining from my (limited) experience. US schools receive funding from many sources, but the budget is set locally (usually once a year) by administrators and approved by a locally elected school board. When administrators make the budget they have to estimate how much money to set aside to pay substitute teachers. The administrators don't know which or how many teachers will get sick so they distribute sick hours to the teaching staff evenly. You can think of sick hours kind of like getting 'shares' in the substitute fund. Now as teachers work for a district over time these sick hours continue to accrue. Basically it means teachers who have worked there for a long amount of time and haven't needed to use the hours have hundreds of 'shares' in the substitute fund. People with a lot of accrued hours can transfer them to other employees. The amount of 'shares' in the substitute fund stays the same, but the 'owners' change. Meaning the giver loses their promise of substitute coverage, but the district can continue paying for both the sick teacher's salary AND a substitute teacher to cover their classroom, AND buy those new crayons they promised. Hope that all made sense.