Ответ:
1. She's got beautiful eyes. They're as blue as the sky.
2. I've got all my books in my bag and I can't pick it up. It is as heavy as lead.
3. I keep my books at school, so my bag isn't heavy. It is as light as a feather.
4 Don't touch me. Your hands are as cold as ice.
5 It's very sunny today and 20°C! It is as warm as toast.
6 David won the 100 metres race. He is as fast as lightning.
Все существительные, с которым сравниваются, неисчисляемые, кроме feather - перо, поэтому артикль перед ними не ставится. Перед sky - ставится артикль the - потому что небо всегда определенное и одно и то же, так же как и солнце.