That’s a wild idea! If you want a single‑cell organism that actually lights up and stays stable, you’re probably looking at a genetically engineered bacterium that expresses a fluorescent protein or a bioluminescent enzyme. First, pick something that’s robust in the environment you’ll put it in – a coffee cup is a nutrient‑rich but variable medium, so you’ll want a strain that tolerates heat and a bit of acid. E. coli engineered to produce GFP or mCherry can survive for a while, but they’ll need a plasmid with a strong promoter and a stable copy number, or you can knock the gene into the chromosome so it doesn’t get lost. Next, to make it *respond* to a tap, you could use a light‑responsive or mechano‑responsive promoter. For example, the bacterial *pLac* promoter can be induced with IPTG, but that’s not touch‑activated. A better trick is to fuse a mechanosensitive channel to a transcription factor – when you tap the cup, the channel opens, lets ions in, and triggers expression of the fluorescent protein. It’s a bit of overengineering, but it works in principle. You’ll also want a reporter that’s bright enough to be seen without a microscope – mCherry or DsRed give a nice orange glow that’s more visible than GFP. Stability is the other big hurdle. In a coffee cup, nutrients will deplete and the pH will drop as the bacteria metabolize sugars, so you’ll need to balance the medium: keep a small amount of growth medium (say a 10‑% M9 buffer with glucose) and maybe a slow‑release carbon source like agarose beads. You could also encode a small “kill‑switch” that stops growth after a set time to prevent over‑growth. If you want it to *stay* stable for weeks, consider using a spore‑forming bacterium like Bacillus subtilis, engineered to produce a fluorescent protein when the spores germinate – the spores will sit dormant until you add a trigger, such as a warm tap. Finally, the “glow” you want to see in a coffee cup is really a fluorescent signal, so you’ll need some light source. A small LED underneath the cup that shines when you tap it could excite the fluorophore, making the color pop. If you want true bioluminescence, you’d need a luciferase system and luciferin substrate in the medium; that’s messier but could give a flicker that’s more alive. So in short: pick a hardy strain, insert a bright fluorescent gene with a mechano‑responsive promoter, keep the medium just enough to survive but not grow out of control, and add a little LED or luciferin to light it up when you tap. It’s a lot of tiny parts to juggle, but the micro‑world is surprisingly forgiving if you give it a good environment. Good luck, and keep that curiosity buzzing!