21 Why do migratory birds go back in the next spring?

Breeding instinct with change in climate

22 Which constituent of the blood carries food materials to all tissues of the body?

White corpuscles

23 What is an instinct?

An immature action

24 How many muscles does a human body have?

173 muscles

25 Why is rest necessary after a good deal of exercise or work?

To neutralize poison that is produced during exercise or work

26 The nerves that carry impulses form the brain are called

Motor Nerves

27 What is a Whale?

A mammal

28 The sky over the earth in fact looks

Blue

29 Can there be light without heat?

Yes

30 One of the sources of the light without heat is

Glow-worm