At the core of every interaction lie three interdependent elements: Form, Behaviour, and Content. They’re not stages, but facets. They all play together when we engage with a system, object, or interface. Two children talk through a pair of paper cups connected by a string, an old, playful, analog communication device. Even in this simple setup, the triad of Form, Behaviour, and Content is at work. It is not just the children interacting but the system itself participates in enabling that interaction.

Form

This is the structure. What you can see, touch, or hold.

1. Two paper cups
2. A taut string connecting them
3. Holes at the bottom of each cup

This physical design isn’t random. The cup’s shape amplifies sound. The string must be tight. The Form gives the interaction its medium and material presence.

Behaviour

This is how the system works. The cause-and-effect mechanics.

1. One child speaks; vibrations hit the cup’s base.
2. The vibrations travel along the string as mechanical waves.
3. The second cup converts them back into audible sound.

If the string is loose, the message won’t travel. If it's wet or frayed, the signal weakens. The system has rules, behaviours that respond to actions.

Content

This is the message itself.

1. A joke, a secret, a silly song.
2. Voice, tone, pauses, even silence.

Content is the meaning that rides on the medium. Without it, the system works, but there’s nothing to exchange.

Not Just People, but Systems Interact

Here’s the key: the two children are interacting with each other, but their interaction is made possible by their interaction with the system, i.e. the cups and string. They shape the system (by pulling it tight, positioning it), and the system shapes their experience (by allowing or distorting the message). The system is not passive; it responds, translates, and mediates. And so Form, Behaviour, and Content form a loop between human intention and mechanical facilitation.

Now let’s take this to the design world

1. Form: Buttons, fields, icons, spacing, this is what the reader sees. It invites action. Good form suggests function. Bad form confuses. Visual clarity shapes first impressions, and often, expectations.
2. Behaviour: This is the choreography. What happens when someone taps, scrolls, drags, swipes? Does the system respond immediately? Does it animate, delay, disable? Behaviour defines rhythm, control, flow. This is interaction design’s core concern.
3. Content: Words, labels, microcopy, messages. Content transmits but content also explains. It reassures. It prompts and guides. Content bridges interface and language. It shapes tone, trust, and clarity.

Good design doesn’t pick one. It balances all three. A clear form with no behaviour stays static. Clever behaviour with poor content leads to confusion. Beautiful content in the wrong form goes unread. Think of them not as silos, but as strands in a braid. Tension in one affects the rest.

Dependencies.

1. Form requires ergonomics. It must suit the body—thumbs, eyes, motion. It must support interaction. A form with no function is decoration. That’s why the principle holds: form follows function. Not blindly, not mechanically—but in service of what the system allows and enables. Design starts with use, not style.
2. Behaviour requires understanding. It must make sense. It must offer control. The reader acts; the system reacts. The feedback loop needs clarity, pacing, and logic. Tap, drag, scroll, each action should feel guided, not guessed.
3. Content requires structure. It needs hierarchy, flow, and emphasis. Good content doesn’t just speak, it arranges. It guides reading, scanning, and decision-making. It makes sense because it’s shaped well.

The Triad in Practice

1. Form invites
2. Behaviour enables
3. Content explains

Together, they make systems usable, readable, and responsive. When aligned, they let the reader act without effort—and feel understood.

---