From Handknit Holidays:
It’s said that mittens keep hands warmer than gloves--there’s more space inside a mitten to trap air, and your fingers can share body heat in a way they can’t when they’re isolated in gloves. Still, Kim Hamlin notes, nothing is colder than having to remove your whole mitten to do a task. Her flap-top mitten is the perfect solution. Flap up, your fingers are kept cozy by thick wool and your own body heat. Flap down, they’re free to tie skate laces or press cell phone buttons while the rest of your hand stays warm.