My teenage son is involved in a live-action roleplaying game 
(imagine people dressing up in costumes and acting out D&D-type 
adventures). They use foam "weapons", and represent throwing 
"magic spells" by tossing little cloth packets of birdseed. For 
uniform packing, my son prefers to use plain white millet seeds 
(which look astonishingly like hemp seeds, but that's another 
story entirely). Recently he borrowed my car to drive himself and 
a few of his friends to one of these adventures. 

Apparently while he was driving, his friends were making seed 
packets in the back seat, and managed to spill a fair amount of 
seeds all over the rug. The next day, he *said* he'd cleaned up 
most of it. I didn't pay very much attention to the back seat; 
mostly I was just in a hurry to drive off and do some errands. A 
few days later he borrowed my car again. When he brought it back 
to me, there was a mostly-empty spring water bottle rolling around 
on the floor, which I promptly got rid of myself. I didn't notice 
that the bottle had leaked onto the rug.

The car sat in the hot sun for a few days, while I continued to 
not pay attention to the back seat. Today, though, I had to haul a 
number of bulky items home, and I went to put them in the back... 
and discovered that the millet seeds, on the soggy wet rug, in the 
warm sunny car, had begun to sprout!

Yes... my VW has turned into a giant Chia Pet.