I don’t have a pool.  Or a gas grill.  But I am impressed at the
display of

backyard ingenuity shown by the guy who devised a tight lattice of copper
tubing to fit inside his gas grill for pool-heating (via).

According to

MAKE, one five-gallon propane tank raises the temp in an 8,000-gallon pool
by 7 degrees.  Even though, as the MAKE commenter points out, gas-fueled
pool-heating is a waste of precious resources, I like it because it’s exactly
the kind of project that my brother would undertake.  To think of sweating
all those joints!  In fact, there’s
a good chance that if he sees this entry, he’ll have one of these put together
for warming my nephews’ plastic pool within a week.

And no, this entry wouldn’t be complete without acknowledging that there are
more conservation-minded, if disgusting and indefensible, ways to heat the pool. I’m just saying.