Combine yeast, sugar, and 1/4 cup warm water in a small bowl.

Let stand for 5 minutes until foamy.

Sift flour and salt into a large bowl.

Make a well in the center.

Add eggs, oil, remaining water, and the yeast mixture to the well.

Gradually incorporate the flour into the wet ingredients.

Knead the dough until smooth and elastic, adding more flour if needed.

Place the dough in an oiled bowl, turning to coat.

Cover and let rise in a warm place for 1 hour, or until doubled in size.

Divide the dough into 3 equal pieces.

Roll each piece into a long rope.

Braid the ropes together.

Place the braided challah in an oiled baking pan.

Cover and let rise again until doubled.

Preheat oven to 375°F (190°C).

Brush the challah with egg yolk.

Sprinkle with poppy seeds, if desired.

Bake for 40 minutes, or until golden brown.