Please please wear your helmet, even for that tiny trip round to the shop.
My cousin decided it was too hot to cycle home and didn't wear his. He was hit from behind by a car blinded by the low sun. He's now just come round from a week and a half coma, only able to blink his eyes and raise his hand.
It looks likely that his best prognosis is that he'll have a 5% recovery, need 24/7 care for the rest of his life, he's only 36.
Please don't leave it behind?