Excitatory neurotransmitter? This question is too vague.
Answer:
Because they can be really really bad for you and sometimes if they get too bad you could die. Or sometimes spread it to someone else.
Answer:
Exactly in the four elements iron, nickel, cobalt and gadolinium, there is interaction between so-called 'unpaired spins'. This interaction ensures that the magnetic moments of atoms can permanently align parallel to each other. The sum of all these small magnetizations forms the nett magnetization of the material
Explanation: