Five Brilliant Brain Teasers For The Week — Time Travel Twist Edition

Tiny stories, big paradoxes. Try not to rewrite history.

🧠 1) The Mysterious Postcard (Easy)

A man receives a postcard in the mail with tomorrow’s date and a single sentence: “Don’t get on the 9:15 — trust me.”

He avoids the 9:15 and later hears on the radio that train had an accident. He calls the number on the card to thank the sender. A voice answers: “You’ll thank me tomorrow.”

How can the postcard writer be right about the date, and why does the sender say “You’ll thank me tomorrow”?

🧠 2) The Birthday Switch (Easy)

A woman is born on January 1st, 2000. She celebrates her 21st birthday on December 31st, 2020, and then celebrates her 22nd birthday on January 2nd, 2021. There was no mistake with dates or calendars.

How is that possible without time travel being obvious?

🧩 3) The Investment Tip (Medium)

An investor claims he made a fortune because a man from the year 2040 gave him a newspaper showing the winning numbers for a series of lotteries over the next 20 years. The investor used the numbers and got rich. Later the investor meets the man from 2040 again — but the traveller says: “I never had that newspaper; I only ever read about you from it.”

Explain how this could happen without anyone lying.

🧩 4) The Crime That Couldn’t Be Prevented (Medium)

A detective time-travels back one week to prevent a robbery that will ruin a small business. He watches the week unfold and, during the crucial moments, shouts a warning that scares the thief off. The detective returns to the present happy — only to find the business still ruined and the robbery recorded in the newspaper.

What likely happened?

🧠💥 5) The Origin of the Song (Hard)

A composer in 2100 performs a haunting melody that becomes world-famous. The melody is distinctive and never previously recorded anywhere. Decades later, a time-traveller from 2150 travels back to meet the composer and, impressed, transcribes the melody and gives the sheet music to a young composer in 2090. That young composer uses it in a famous song, which inspires the 2100 composer to write the haunting melody in the first place.

Where did the melody originally come from?

✅ Answers & Explanations (no peeking earlier next time!)

1) The Mysterious Postcard — Answer

This is a bootstrap / causal loop situation. The postcard came from the man himself tomorrow — after he avoided the 9:15 and lived to mail it. When he calls the number, the person says “You’ll thank me tomorrow” because the caller is the man in the future who will send the postcard tomorrow after seeing today’s outcome. The postcard’s existence is a closed loop: the information (don’t take the 9:15) has no clear origin beyond the loop.

2) The Birthday Switch — Answer

There’s no sci-fi time travel here — it’s about time zones (or traveling across the International Date Line). She was born in one time zone (or on a plane/ship crossing the date line) and later spent New Year in another time zone where dates differ.

Example: She celebrates her 21st on Dec 31 in one time zone (local date still Dec 31) and then crosses into another zone where it’s Jan 2. The dates line up without an error in calendars.

(Another simple non-paradox explanation: she celebrated early with friends on Dec 31, then had an official party on Jan 2 — the riddle’s point is to think about shifting dates, not a literal time machine.)

3) The Investment Tip — Answer

This is another bootstrap/information loop (also called an ontological paradox). The traveller from 2040 didn’t originate the newspaper; the newspaper’s set of numbers exists in a closed loop because the investor used them to win and then (later) those same winnings or recordings cause someone in 2040 to have the newspaper that references the investor. In plain terms: the information about winning numbers circulated in a time loop — the traveller learned of the investor from the future material that was itself a product of the investor’s winnings. No one lied; the newspaper’s provenance is circular.

4) The Crime That Couldn’t Be Prevented — Answer

This shows a self-consistent / predestination outcome. By warning the thief, the detective changed the thief’s behaviour in a way that still produced the robbery’s recorded effects — for example, the thief fled, but in the escape caused a different chain (vandalism, insurance fraud, or theft by an accomplice later) that still ruined the business. Alternatively, the detective’s warning may have altered, not prevented, the timeline: his intervention became the very cause that led to the reported outcome (a common predestination twist). In short: his attempt to prevent the event was part of the causal chain that produced the event’s recorded consequence.

5) The Origin of the Song — Answer

This is the classic bootstrap (or ontological) paradox: the melody has no original composer in the linear sense. It exists because it was passed back and forward in time — each person’s inspiration came from the melody they received from the future. The tune is a self-originating loop: it exists in the timeline but lacks a point of origin. Philosophically puzzling, but logically consistent within many fictional time-travel rules.