Answers! 

1. Monday: There are not gray dots in the grid. However “ghostlike” gray blobs are perceived at the intersections of the white lines. The gray dots disappear when looking directly at an intersection. This illusion can be explained by a neural process happening in the visual system called lateral inhibition (the capacity of an active neuron to reduce the activity of its neighbors) 
2. Tuesday: 3
3. Wednesday: Stare at the yellow stripe in the middle of the fish in the picture below for about 10–20 sec. Then move your gaze to the fish bowl. 
4. Thursday: The two horizontal lines are of the same length, even though the one at the bottom seems longer. As you know, the visual angle gets smaller with distance, so the brain automatically perceives objects at farther distances to be bigger.