A liquid level indicator goes on the side of a cup or glass, and has sensors that let you know when the liquid gets to the top. You can read more about it here.
Panel 1: Sigma’s kitchen. Sigma is holding a liquid level indicator in his hand. Looking bummed, he says “Ugh…my liquid level indicator’s out of batteries…”. Rho, standing next to him, waves his hand and says “That thing that beeps when you fill your mug? Dude, I can do that.”
Panel 2: A close-up of Sigma, narrowing his eyes with a skeptical look. He says “what”. Rho (off-panel) says “Dude, just fill it in!”, to which Sigma replies “…fine”
Panel 3: On the foreground, a water kettle pouring water into Sigma’s cup. On the background, Rho clenches his fists looking at the cup. He says “Easy…”
Panel 4: Same as the previous panel, but now Rho opens his mouth wider and says “Aaaaa—”
Panel 5: A close-up of Rho, clenching both of his fists, screaming “AAAAAA” in big, bold letters.
Panel 6: A close-up of Sigma, his eyes closed shut, still holding the electric kettle with an expression of huge discomfort. On the background, the “AAAA” screams from Rho continue from the previous panel, in big bold letters that gradually get more and more red.