Interesting background to this experiment! This is a developmental psychological test performed (initially by Jean Piaget) and later by Judith Samuel and Peter Bryant in 1983 to test at what age children learn to conserve what they see. Originally Jean Piaget, a Swiss bloke, performed this experiment with 3 different tests. He would, with the water, show them the two equal glasses, ask which has more, or if they're the same, then change the beaker of one, and ask again. He also did this with 2 balls of Plasticine, changing one's shape after initially asked, and would ask if they're the same mass again, and with rows of counters. He would lay out two rows of equal counters, ask if they are the same or one has more, then spread out one and ask again.

Samuel and Bryant criticized this experiment by that many of the younger children (5-6 years) would get it wrong because they felt that by being asked twice they got it wrong the first time. Samuel and Bryant performed these tests 3 ways. Once exactly as Piaget did, by asking, letting the children see the transformation, then asking again. Then a one judgement test to a group who had not seen the transformation and only two different looking items while asking once, and finally a fixed array test where they were sat down, saw the equal looking items, then saw the transformation and were asked one question at the end whether they were the same, or if one had more/was different.

Typically, children around 5 to 6 would get many wrong, but around 7 years they were showing that many had learned to conserve what they saw (especially in the fixed array, as well as the initial test) and got the answers right with a clear explanation as well as a 7 year old can explain.

tl;dr - little kids find it difficult to think they are the same if they're asked twice

EDIT: I left out full results, left it very vague and made it seem as though 7 is the "concrete" age at which children begin to conserve, so sorry for any confusion. You can read more here or if you're not satisfied... I'm not stopping you from looking it up yourself.