You put the cylinder on a table, you place the block over it and you cover the whole with a second cylinder.
You borrow four coins (1 Euro, 50 cents, 20 cents o even 5 cts) that you place on the top cylinder. The coins do not fall.
Ask a spectator to gently tap the edge of the cylinder and a coin will pass through the cylinder.
The three remaining coins are poured on the table.
You try again the experiment and reintroduce the coins in the top cylinder. A second coin passes through the block and falls on the table. The audience can clearly see the brass block between both cylinders. However the two last coins pass through this block.
You can give the material to the audience for examination.