Seeing that 2/4, 3/6 and 1/2 are the same number wearing different clothes.
Equivalence is the single idea the rest of fractions rests on. Adding, subtracting and comparing all quietly assume a student can see that two different-looking fractions are the same size. Matching them by sight, again and again, builds that faster than a rule about multiplying top and bottom ever does.
After the maths shelf covers fractions.