Candles were often used as clocks in the middle-ages - they burn at a steady rate and can measure time reasonably accurately.

There are a couple of ways we can make a candle burn for longer.

• One is by using a candle with more wax in it, i.e. one that contains more fuel energy for burning.
  
  
• The other is by placing the candle on the fast-moving train and observing it from the platform - the candle is as subject to time dilation as described in the previous section, and lasts for longer.

(NB. Someone on the train would observe their candle as having its normal lifetime, and would see a candle on the platform as having an extended lifetime.)

Explanation

Why do these candles last longer?

In the first case it is obvious - the candle contains more fuel energy and therefore takes longer to burn.

In the second case, let us assume that the candle must likewise contain more fuel energy, i.e. the total energy of a moving body is greater than the total energy of a stationary body.

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