My user: My grandmother, Nadia. She has arthritis in both hands. Every morning she waters the eight plants on her windowsill, and by the last two the can is too heavy to hold steady.

The motion: Get water from a jug into each pot along the windowsill, without Nadia lifting anything.

How I could do it: Three ways. (1) The arm carries the watering can to each pot and tips it. (2) The can stays put and the arm moves the pots to it. (3) Skip the can. Run a thin tube from the jug and have the arm move just the end of the tube from pot to pot. I went with 3.

Why: Carrying the full can is the exact part that hurts her, and option 1 still needs somebody to load a heavy can, so it automates the wrong thing. Option 2 turns one lift into eight, which is worse. Option 3 removes the lifting completely. It is also the safest thing to get wrong: if the arm knocks the tube over it spills a trickle, and if it knocks a full can over it puts a liter of water on the power strip under the window. Nadia told me she would rather it be slow than have to stand there watching it, so I am not designing for speed.

What could go wrong: The far end of the windowsill is about 45 cm from anywhere the arm can sit, and the arm only reaches about 40 cm. The last two pots are out of range at any angle, so the tube has to be long enough to close that gap on its own. I want to shift the base closer, but then it may not clear the window frame, so I am going to test which pots it can actually hit before I cut the tube.

My criteria: 1. Nadia does not lift the can at all · 2. Every pot the arm can reach gets watered · 3. The tube stays clear of the power strip

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