В отличие от союзов, союзные слова _ членами предложения отвечают на какой-либо вопрос и являются

Shop-assistant: What can I do for you?

You: I would like to buy something for my little sister.

Shop-assistant: Well, how old is she?

You: Tomorrow she will be 4.

Shop-assistant: Really? What about this pretty doll?

You: Oh, no! There are ten or fifteen dolls in our flat. They are everywhere: on the sofa, on the chair, on the computer...

Shop-assistant: Sorry, it's awful. Well, what about this drum?

You: Oh, no! It's so noisy!

Shop-assistant: Let me see. Oh, look! A teddy-bear.

You: Never! She has seven or nine taddy-bears.

Shop-assistant: Well, I don't know...May be puzzles?

You: Yes! Wonderful! Two ... No! Three! How much do they cost?

Shop-assistant: Three dollars.

You: Here you are.

Shop-assistant: Well,  your change...

You: Thank you.

Shop-assistant: You are welcome!