If today it was 0° and tomorrow was twice as cold, how cold would it be tomorrow?

At first glance this question seems pointless. However, there are two ways of finding an answer to this question which, I hope, make sense. As the question does not define the temperature unit let’s assume it is degrees Celsius. Now you could say that 2×0 °C is still 0 °C. Well, you’re right. But… according to the ITS-90 (the International Temperature Scale of 1990) the triple point of water (i.e. the freezing point of water) is 0.01 °C. This point is used to define the scales of thermometers, so I think I can use it for my answer without getting egged.
So, if today it was 0 °C it would actually be 0.01 °C and if tomorrow it was twice as cold, it would be 0.02 °C. Not that you’d feel any difference, but still.

Do you get your money back if a taxi drives backwards?

Of course you do. Whenever I take a taxi I tell the driver to drive backwards, so he has to pay me. That’s how I earn a living. Quite smart, isn’t it?
If only…
The first taximeters worked like tachometers for bicycles. They were used on hackneys, measuring the turns of the wheels and thereby calculating the fare. Today they work differently. When today’s taximeters are activated they automatically show a basic fee to which the extra fees for the distance traveled and the time waited (at traffic lights) are added.
Once the taxi is driving you can compare it to riding a bike: the tachometer does not count the distance when the wheels turn backwards. It’s the same with cars. Whenever you drive backwards the odometer does not register any changes. So, no, there is no chance of getting your money back when the taxi drives backwards.
Even if the odometer counted the distance covered when driving backwards, it would be to your disadvantage as all the kilometers would still be added to the total distance.

Why are carrots more orange than oranges?

Carrots and oranges are orange because they contain β-carotene. Carotene is a photosynthetic pigment which is important for photosynthesis and for the color of various fruits and even animals. Consequently, the intensity of the color of oranges and carrots must have to do with amount of carotene of both. While oranges contain approximately 0.5 mg/kg of β-carotene, carrots contain around 80 mg/kg. So a carrot contains 160 times more β-carotene than an orange. Therefore, they are more orange than oranges.

Why does everyone see that I am drunk but not that I am thirsty?

Being drunk comes along with certain signs, e.g. swaying, slurred speech etc. These symptoms become obvious at around 0.5 ‰. By-standers are able to observe and identify the signs of drunkenness, i.e. a drunkard is obvious.
Someone who is thirsty does not display such obvious signs. When you lose around 10% of fluid you will have a dry mouth and impaired speech. These signs, however, are not obvious to by-standers. E.g. a bar keeper will never notice your dry mouth and impaired speech could also be due to having had too much to drink.
Even when you’re dehydrated and lose consciousness no one might suspect you’re thirsty. People become unconscious for a lot of reasons.
All in all I’d say the signs of being drunk are more obvious and easier to identify beyond all doubt than those of being thirsty.