Why do galaxies collide?

Well, they collide due to their 'mutual gravitational attraction' in other words they collide because gravity draws them together. However in general galaxies are moving away from each other at an ever increasing rate, basically because of the Universe expanding at an ever increasing rate. This can be modeled simply by drawing stars or dots (imagine these as galaxies) onto a deflated balloon then blowing up the balloon. An idea that has sprung from this is the Red Shift. This is where astronomers have found that the further away a star is the more it's light has been red shifted. So this tells us that distant galaxies are moving away from us and that the further away a galaxy is the faster away its moving away from us.

The Red Shift is an example of the Doppler effect, named after the Austrian physicist Christian Doppler who proposed this theory in 1842 in Prague. You may have noticed that as a police car comes towards you the siren becomes high pitched and low pitched as the car drives away. This effect is where there is a change in frequency and wavelength. So when a source moves towards an observer the observed wavelength decreases and frequency increases and vice versa. In terms of Astronomy the results of the Doppler effect are used to identify if a nearby star is what astronomers call a binary star. This is two stars orbiting around a 'common center of mass' which is where the relative position of the distributed mass adds to zero. The brighter star is called the 'primary star' and the other the 'secondary star'

In our own 'Local Group' of galaxies for example some of the galaxies are moving away from us and some are moving towards us. Take our nearest galaxy the Andromeda galaxy for example which is currently moving towards the Milky Way at 402, 336 km/h and is due to hit us in 4 billion years.

Why does skin stick to frozen metal?

If you put your tongue on any frozen metal, the high conductivity of the metal removes the heat from your tongue much faster than your body can supply it. This makes the saliva freeze into ice very quickly. This ice creates hydrogen bonds between the metal and your skin, just like water does, so the whole surface bonds together and resists as you pull away. This same thing will happen with your fngers because your fingerprints provide the roughness needed and your fingers always have some moisture due to sweat. However very smooth, dry skin won't stick. 

Why does a mint make ice cream even colder?

All mints contain the chemical menthol, which binds to the TRP-M8 receptors in your skin which sense cold, the temperature doesn't change it's the menthol that causes false cold signals to be sent to your brain. This feeling is intensified because the ice cream also sends cold signals to your brain and this increases the number of cold receptors that are firing at once. This is similar to the reaction caused by chilli which is caused by the chemical capsaicin which binds to the TRP-V1 receptors for heat. 

Can animals be allergic to humans?

Yes, they can. Scientist's don't know much about allergies with wild animals but it has been known that dogs and cats develop allergies to their owners. Like when humans are allergic to animals, the main culprit is the dander or dandruff that falls of us. Around us everyday there's dust that contains varying amounts of dead skin. When cats are allergic they show symptoms such as coughing, wheezing and shortness of breath. Dogs show symptoms such as itching, sores and skin infections. This problem is increasing a lot because we are steadily domesticating our pets where they are continuously exposed to human allergens. 

How do scientists communicate with deep space probes?

Communicating with something over 11.2 billion kilometres (7 billion miles) away is no mean feat. It involves a huge network of worldwide radio antennas. These are strategically placed across the Earth so there will always be one that the probe can send signals to, 

Likewise space probes (e.g Pioneer 10) need radio antennas to send pictures, weather information and much more to. However power constraints mean the the probes transmit very weak signals that get weaker the further away the probe gets. Antennas have very large dishes to capture the probe's signal. Yet greater amplification and noise reduction is needed to boost the signal to a readable level.   

How do gorillas raised in captivity know what's safe to eat in the wild?

By instinct. Similar to how we have evolved to enjoy the taste of the safe foods and dislike the poisonous ones, gorillas have done just the same. In the wild mountain gorillas eat leaves,shoots and stems so much so an adult male can chomp through a massive 18kg of vegetation in a day. In experiments gorillas were given over 2000 pairs of foods and they then had to choose between them. Throughout the experiments they demonstrated consistent food choices in high sugar fruits with a high ratio of sugar to fibre, this is even the same using pictures of the fruit!