A penis has two states which it periodically transitions between: Flaccid and erect. This transition is not instant, and the penis may exist in a state of quantum uncertainty between these two phases, exhibiting some characteristics of both states for several minutes or more.
Most of the time, the penis will be flaccid. A penis in this state is soft, squishy, floppy, stretchable, twistable, bendable. It doesn’t really hold any particular state or position. (It should be noted that many attempts to make use of or demonstrate the amorphous qualities of the penis may cause state change to erect.)
If the penis has a foreskin, it will usually cover most or all of the head in this state.
While flaccid, the penis is generally not all that sensitive and it can be handled to some extent without resulting in a sensation of pleasure or a state change. This quality is frequently taken advantage of: Most of the time, a penis can be touched to direct a flow of urine or can be adjusted to a more comfortable position inside clothing without fear of triggering an erection.
However, if a flaccid penis is deliberately handled in certain ways, it will often cause a change of state.
A fraction of the time, and usually only for relatively brief periods, a penis will undergo a transition to a different form. Rather than being soft and floppy, it becomes harder and more rigid. This is known as an erection. It is caused when blood fills some spongy areas within the penis and is prevented from draining, similar to inflating a balloon.
During an erection, a penis is typically rigid enough to be inserted into something without losing its form and folding back on itself. This feature is required for many sexual activities and masturbation techniques.
While erect, the motion of a penis is severely limited, and the degree of the erection may limit the motion even more. It can usually be moved from the lower torso to somewhere around perpendicular to the body/straight out without much resistance, but can only be moved about 30 degrees side to side before it becomes uncomfortable.
An erect penis will often still have some give to it, although it is no longer completely spongy. Although often referred to as a “boner”, it is not as hard as a bone. The skin on the shaft of the penis will glide smoothly, unattached to the structures within.
If a penis has a foreskin, it will often pull back and reveal most or all of the head in this state.
An erection can be caused by many things. Some common causes (but by no means a complete list) are: Physical stimulation (intentional or otherwise), sexual thoughts, taking part in or observing a sexual situation, pressure on the perineum, a full bladder, exercise, and a butterfly flapping its wings in Tokyo. They can also happen entirely on their own while sleeping.
While erect, urination becomes difficult, if not impossible. This is due to some internal valves closing off, likely to prevent urine and semen from mixing. Even if it’s not completely impossible, it can be awkward, because most urine receptacles are down, and an erect penis can be moved in a number of directions, but down is the one it least wants to go in. It is a cruel twist of fate that many people with a penis will frequently wake up with both a full bladder and an erection.
An erection by itself is not necessarily an indication of sexual attraction, interest, desire, or thoughts.
While erect, a penis usually becomes more sensitive. Some areas become more sensitive than others. If stimulated in the right way for long enough, this can often lead to sexual pleasure and possibly orgasm and ejaculation.
If an erect penis is stimulated, it is often possible to prolong the erection indefinitely. If an erect penis is left alone and is not stimulated, it will often revert back to the flaccid state within a short time. This also is a common occurrence shortly after an orgasm.