Learn Spanish (Latin America): ¿Bailas? Sí, bailo.

Spanish doesn't use helper verbs like "do" or "does" to ask questions! Instead of saying "Do you dance?", you just use the verb for "You dance?" (¿Bailas?) and let your voice go up at the end to show it's a question.

How do people know it's a question if there's no "do" and no subject to swap around?

When speaking, it's all about intonation. You raise the pitch of your voice on the last syllable of ¿Bailas? to make it a question. When reading, the upside-down question mark (¿) gives it away instantly!

Why are the words tú (you) and yo (I) missing?

In Spanish, the verb endings already tell us who is doing the action. The -as in bailas means "you", and the -o in bailo means "I". Because it's so clear, native speakers usually drop the subject pronouns tú and yo unless they want to add special emphasis.

What is the exact purpose of the upside-down question mark (¿)?

Because Spanish questions often use the exact same word order as regular statements, the opening question mark (¿) acts as a reading guide. It tells you to start using a questioning tone of voice right from the very first word.