There are two different scales of Lego brick. The normal size and the much larger Duplo size (shown).
These are incompatible in terms of joining the two types together.
Today’s invention is therefore a Lego/Duplo interface brick.
The underside would be ‘female’ and allow accommodation of Duplo ‘male’ brick surfaces. The top surface would be male and accommodate the ‘female’ underside of the smaller Lego bricks.
In this way, a child growing out of Duplo could still use their old bricks with the finer scale Lego ones.