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We use comparative adjectives when talking about 2 things (not 3 or 10 or 1,000,000 things, only 2 things).

Often, the comparative adjective is followed by "than".

Look at these examples:

• John is 1m80. He is tall. But Chris is 1m85. He is taller than John.
• America is big. But Russia is bigger.
• I want to have a more powerful computer.
• Is French more difficult than English?

If we talk about the two planets Earth and Mars, we can compare them as shown in the table below:

Earth Mars
Diameter (km) 12,760 6,790 Mars is smaller than Earth.
Distance from Sun (million km) 150 228 Mars is more distant from the Sun.
Length of day (hours) 24 25 A day on Mars is slightly longer than a day on Earth.
Moons 1 2 Mars has more moons than Earth.
Surface temperature (degrees Celcius) 22 -23 Mars is colder than Earth.
Although we use comparative adjectives when talking about two things (not three or more things), in fact one or both of the things may be a group of things.
• Mt Everest is higher than all other mountains.
Here, we are talking about hundreds of mountains, but we are still comparing one thing (Mt Everest) to one other thing (all other mountains).