Because the two pipes are connected, and the shower is farther down the line than the toilet. So when you flush, you are hearing the water from the toilet pass by your tub. This is also why you will have sewage in your tub if your waste water line is extremely backed up.
They share the same drain system. Be thankful that flush water etc from the toilet does not bubble in the bath or shower. But it is a badly designed system, it shouldn't happen.
Check to see if the tank is full... if it is check why the pump is not emptying it. If it is empty then there is probably a clogged or missing vent.
In the toilet
A ceiling heat lamp should be placed over an empty space so that it doesn't overheat anything in the room. Putting the heat lamp over the shower rod could start a fire and over a toilet seat could make the seat too warm and burn someone.
its empty. nothing comes out of it
It is not a good idea to exercise on an empty stomach. There are no nutrients to fuel the exercise.
It will not harm your gerbil, if you give it an empty toilet roll tube. It will probbly go in and out of it for a while, and move it around it's cage; It will then chew it up into small pieces, to mix in with it's bedding.
Some popular designs for wet rooms are created with tiles, different showers such as glass or curtain, also bath installations. Wetrooms are most commonly known to be for en suites, shower rooms or in general washrooms this could also include a simple shower room with renovation including a toilet and sink to fill empty space left.
by pouring it down the toilet
it gets really really happy and likes bubble gum icecream
it really depends what type of toilet roll holder you have but for most its this: take the little attach thingy out then swap the empty one for a full one and push it back in
A mainline stoppage.
You will be on the toilet for most of the day. Bring a novel. Oh don't forget the major painful cramping and the sore behind.