1 Yes, the horizontal lines are straight.
2 They are both the same size
3 They are both the same size.
4 Yes, it is a square.
5 The central disks are the same size.
6 The table tops are exactly the same size and the shadows
7 The card should flip-flop between spine towards and spine away.
8 All 3 matches are the same size.
9 The diagonals are equal in size.
10 The stripes are equal in size.
11 The rows of dots are equal in size.
12 It is either a hexagon or one of 6 cubes. The top face
depends on which cube is chosen.
13 The lines are continuous.
14 The rectangle is a square.
15 The central squares are the same size.
16 The horizontal lines are parallel.
17 The spot can be either on front face, back face or floating somewhere between.
18 The vertical lines are parallel.
19 No, this impossible triangle cannot be made.
20 There is one diagonal line (split into two by the horizontal black bar).
21 There are 6 or 7 cubes.
22 The long diagonal lines are parallel.
23 The ring is a perfect circle.
24 There is no white triangle, but most folks see one.
25 There are 2 pencils and 2 part-pencils.
26 This is op art in action.
27 The planks are straight.
28 This figure flip-flops, so the front face changes.
29 There are 2 planks and 3 part-planks.
There are several reasons for this.
We see in 3D and that means perspective is important to us. This ability enables us to judge the distance objects are away from us. Many of the illusions presented here use perspective elements, usually lines centred on a vanishing point, which mislead us (example #8 receding matches)
Several perspectives are used in the same drawing (example impossible triangle #19).
There are some situations which the brain simplifies and draws it's own conclusions (example #24 white triangle).
There are situations where the brain is overoaded and struggles to cope (example #26 op art in action).
There are many more illusions, if you come across good ones, tell me about them.