A man must say three things in his home on the eve of the Sabbath just before dark: ‘‘Have you tithed? Have you prepared the eruv? Kindle the [Sabbath] light!’’ If there is a doubt whether it is dark or it is not dark, we may not tithe defi-nite[ly untithed produce], nor may we immerse vessels, nor may we kindle the lights; but we may tithe demai, make an eruv, and insulate hot food.