Step 1​

​There are two glasses, one filled with red liquid (labelled R in the diagrams) and the other with blue liquid (labelled B in the diagrams). The amount of liquid in each glass is the same.

Step 2​

​​ ​Transfer some red liquid from R to B, either by pouring or spooning. (There is no "trick" to this puzzle, so let's mention that no liquid gets spilt or evaporated,.)

Step3​

​​ ​Mix the red and blue liquid in glass B. (It doesn't matter whether you mix the liquids thoroughly, just a little, or not at all.)

Step4​

​​ ​Now pour liquid from glass B into glass R so that both glasses again have equal amounts of liquid.