Do I have to sit still to practice mindfulness?
No. Stillness is one training environment, not the whole practice. Walking, lifting, stretching, cleaning, cooking, and eating can all become mindfulness practice if you bring deliberate attention to what is happening: sensations, breath, surroundings, urges, emotions, and thoughts, without immediately chasing or resisting them.
Movement can be especially useful if you feel restless, anxious, or disconnected from your body. A mindful walk may be more accessible than forcing a long seated meditation. It can also train useful real-world skills, because life rarely asks you to be aware only while sitting quietly with perfect conditions.
But seated, quiet practice has a distinct value. When you remove extra stimulation and focus on something relatively simple, such as the breath, you train attention in a more targeted way. The breath is minimally "interesting", which is part of the point: You learn to notice distraction, return attention, and observe experience without needing novelty or movement to hold your mind in place.
The analogy is weight training. A physically demanding job can build strength and work capacity, but a dedicated lifting session lets you train specific muscles and movement patterns more precisely. In the same way, mindful movement is valuable, but a dedicated meditation session is a more focused drill for awareness, concentration, and emotional regulation. The best approach is usually both: use seated practice to build the skill, and movement practice to carry it into life.