3/5 or 2/3 — which one is bigger? When the denominators are different, this can be surprisingly confusing. Build two fractions yourself with the sliders and fill up the bars — see which one rises higher, and the answer becomes clear. Denominators go up to 100, so you can try extreme cases too.
Why is it confusing when the denominators are different? Think of cutting a pizza into 3 slices versus 5 slices. It's not just the number of slices that's different — the size of each slice itself changes too. That's why you can't just compare "2 out of 3 slices" and "3 out of 5 slices" by looking at the numbers alone.
That's where finding a common denominator comes in. Once both fractions share the same denominator, the slices are the same size, so comparing the numerators alone gives an accurate answer. This activity automatically finds a common denominator using the least common multiple of the two denominators.
The thin lines inside the bar only show up when the denominator is 20 or less. With 100 slices, the lines would get so crowded they'd be hard to read — in that case, just compare how high each bar is filled.