resemble the object or action they represent, e.g: a hand washing sign.
have a direct link to what they are indicating.
e.g: medical symptoms are a sign of an illness
smoke is a sign of fire
the link is causal, as the fire causes the smoke, and the illness causes the visual symptom
may not look like, or have a link to what they represent, so we have to learn what they mean through constantly being shown a link between the two.
e.g: the apple logo means nothing on its own but because we have seen it in context so many times, next to the macintosh branding and on the products, we now know it as a logo as well as a symbol.
Signs can only be interpreted by looking at its at its context. The simpler they are, the more ambiguous the meaning, so its surroundings are needed in order to understand the message.
A circle is so simple, that it could represent many things. However, if a point is added, it becomes another, more complicated shape, and so it can resemble a drop of liquid (even though a raindrop is spherical).
A black, red or blue dot does not mean anything, but does connote certain ideas based on the colour. If the colour is then put along with another sign, such as the shape of a raindrop, it gives it a context, and so it can now resemble, and represent something.
The black could mean oil, the blue; water, and the red; blood.
The signs on toilets are iconic, as they look like men or women. However, if they are not placed somewhere specific, such as a toilet door, they do not have enough meaning to convey a message. On the door, they can mean; 'mens room' or 'ladies room', or which we have learnt tends to mean toilets.
Then, with both symbols together, this then means unisex toilets.
If these symbols were simply on a wall, they do not have enough other signs around them to signify a meaning.
Here, I have combined symbolic and iconic signs (fire and smoke) to represent the idea of indexical signs (which have a causal link to what they represent). So, in this case, the smoke would be an indexical sign for fire, and fire is an indexical sign for combustion.
The red circle with a cross through it would mean nothing on its own, without another sign within it, making it another symbolic sign.