When you see this signal, the other diver is OK and wants to know if you are
 
OK, so youre starting on the theory lessons, either with others in the classroom or on your own at home using CD-ROM, video, DVD or a website. You can look forward to a number of hours of lessons, usually split into one-hour sections, and a series of short quizzes to check that you have understood and remembered what you have been taught.
You will learn about the underwater environment and how this affects your body, about diving equipment and the skills you will practise in the water.
There is some mathematics involved, but nothing too challenging.
For example, you learn about Boyles Law, which deals with the relationship between pressure and volume. Suppose you have a balloon with a volume of 1 litre. Double the pressure outside the balloon and its volume will be squashed to 0.5 litres. Halve the pressure outside and the balloon will expand to 2 litres - if it doesnt pop first. Thats about as complicated as the maths get when youre learning to dive.
The lessons may not be difficult, but you will be expected to put in the hours and study them seriously.
In the confined and open water training that follows, you will be taught, and then practise, various diving skills.
Confined water means a swimming pool or sheltered shallow water off a beach no deeper than a pool. Open water means deeper water, more like where you will be diving once qualified. Youll practise all your skills in confined water before going on to open water diving.
You do need to be able to swim. For some training agencies you just sign a form stating that you are capable; others expect you to do a basic test, swimming a few lengths of a pool, treading water and making a duck-dive below the surface.
So you dont have to be a competition swimmer, or even to use proper strokes - dog paddle would do. The important thing is that you are a happy and comfortable swimmer.
All the diving skills break down into smaller skills, so you learn everything in stages.

EAR CLEARING

Many people new to diving worry about the feeling of pressure on their ears. You may have noticed this when simply diving to the bottom of a swimming pool. The water pushes on your ears, and going deeper can be painful. There is a simple solution, however: you compensate the pressure inside your ears and everything is balanced again.
The usual trick is to pinch your nose and blow against it gently. This pops air into your ears and everything is balanced to the water pressure. You might have done this on an aircraft during its descent. If not, try it now and feel your ears pop.
During training your instructor will gesture to remind you when to clear your ears, though you will soon develop a feel for it and know when to do it without the instructor having to remind you. Any problem and the solution is equally simple - go up again.

BUOYANCY CONTROL

This skill is all about being neutrally buoyant, so that you can swim along comfortably at any depth without tending to float or sink. You do this by adjusting the amount of weight you carry on your weight belt before the start of a dive, and by adding and releasing air from your buoyancy compensator jacket (BC) during a dive.
You also learn the technique for fine-tuning your buoyancy by breathing more deeply or shallowly.
There are two main exercises that teach this skill. A fin pivot means lying face down on the bottom and dribbling air into your BC until you can breathe in and rise up slightly, then breathe out and sink again. Its the volume of air in your lungs that controls your movement. This mastered, you move on to hovering, adjusting your buoyancy so that you can float clear of the bottom but without breaking the surface. You choose your depth and stay there by controlling how deeply you breathe.
The important thing is to learn to do this without flapping your arms or legs, so your instructor may get you to cross your legs and fold your arms while practising. Its a bit like underwater yoga.

PROPULSION

Youll learn how to move your feet up and down with straight legs to provide an effective fin kick. Beginners are often surprised to learn that they dont need to use their arms.

REGULATOR REMOVAL, RECOVERY AND REFIT.
The regulator is the piece of equipment from which you breathe. You have to learn how to take it out of your mouth and put it back in while you are under water.
By the end of your training you will be able to remove it, throw it behind you, relocate it and put it back in. Knowing you can do this easily makes you feel more secure.

MASK REMOVAL AND REFIT AND CLEAR
You will also learn how to take your mask off and put it back on while under water. Once its back on, you have to clear the water from it. You begin to learn this skill by leaking a little water into your mask and clearing it, and progress until you can fill the mask and clear it.
So how do you get the water out of your mask Its easy. Your nose is inside the mask, so simply by breathing out through your nose you fill your mask with air and push any water out of it. The technique involves holding your mask at the top and tipping your head back at the right angle, but your instructor will show you how to do it.

OUT-OF-AIR DRILLS
One of the most dangerous things that could happen to you under water is your regulator failing or running out of air. Neither is very likely to happen. Regulators should be serviced regularly and are extremely reliable, and if you keep an eye on your cylinder pressure gauge you should never run out of air. Even so, accidents do happen, so being prepared for anything is the safest course.
Divers always work in pairs, or buddy teams, so that should something go wrong, your buddy can help you out and vice versa. Whichever agency you train with, you learn how to share an alternative air source with your buddy.
The regulator from which you breathe will be fitted with two second stages - the bit that goes in your mouth. You use one to breathe through, and the other is there
in case your buddy needs it. The spare is usually known as an octopus regulator.
You will practise swimming to your buddy, removing your own regulator and breathing from your buddys octopus. You then swim together to the surface and make sure that you cant sink again by fully inflating your BC and/or removing your weightbelt. You get to do this both as the receiver - the diver who is out of air - and as the donor.
Through your training you will repeat this exercise while simulating greater depths by swimming along before ascending, then in the open water actually ascending from depth using an octopus.
More rarely taught these days is a variation called buddy breathing, where a pair of divers take turns to breathe from a single regulator second stage.
Another skill you may be taught for out-of-air situations is simply swimming to the surface in a controlled emergency swimming ascent. The key thing is to remember to breathe out all the way up, because the air in your lungs will be expanding. Remember the balloon example earlier!
In very shallow water, a swimming ascent can be the most sensible thing to do, but as water gets deeper the prospect becomes riskier and it is safer to use your buddys octopus.
During training the depth involved is less than the width of a standard swimming pool. Your instructor will build you up to the final swimming ascent exercise by finning along in the shallow end of a pool or confined water, increasing the distance gradually as your confidence grows.

DIVE PLANNING AND CONDUCT
There are lots of other little skills to learn during your training course. We cant detail them all here - thats what the course is for.
Despite the emphasis on emergency drills, the main thing you will learn is how to plan and then conduct a dive. You will learn how to read decompression tables that tell you how long you can safely dive at a particular depth, and to monitor your watch, depth gauge and cylinder pressure gauge so that you can ascend within the limits set by the tables with plenty of air left in reserve (computers come later).
Aren't you itching to get started

Practising
Practising buoyancy control - you should be able to hang at a chosen position in the water
Take
Take out the regulator, retrieve and replace
A
A controlled buoyant ascent
Should
Should your regulator ever come out of your mouth, you learn a simple way of retrieving it fast
Learning
Learning to lift a buddy under water