oil and water

you have two identical containers (jars).

in one you have some oil and in another an equal amount (volume) of water

some oil is poured into the water jar and thoroughly mixed in

whilst still mixed, an equal amount of the mixture from the water jar is scooped out poured back into the oil jar



which now has the greater ratio:
oil in the water jar
water in the oil jar?

yes, you can tell

it is interesting for students to model this, using counters (or people)
around 8 of each is a reasonable number to use
fairly randomly pick some 'oil' and put it into the 'water'
then pick the same number of counters (randomly if possible) and put them in the 'oil' jar

when you have a fairly confident answer, try to explain it