The quote had two prices#

You saw one number. There were two.

bid — someone will buy from you

ask — someone will sell to you

The ask is higher.

Say the quote was#

$39.96  bid
$40.00  ask

You paid the ask.

Change your mind one second later and you receive the bid.

So at the instant you bought#

$40.00 − $39.96  =  $0.04

$0.04 / $40.00   =  0.10%

No news. Nobody was wrong about anything.

Then the platform charges you#

$5 in  +  $5 out   =  $10

$10 / $10,000      =  0.10%

One round trip#

0.10%  +  0.10%  =  0.20%

$20 on $10,000.

In and out. No opinion about the market.

Twenty dollars is not interesting#

What makes it week 2 is that it recurs.

And recurring costs compound against you

exactly the way returns compound for you.

Leave it alone for forty years#

Total drag 1% per year:

0.99^40  =  0.669

Not to a crash#

Not to a bad decision.

Not to anything that appeared in a newspaper.

To a number so small it is usually quoted as reassurance.

Now use your own number#

Four round trips a year, at the 0.20% we measured:

4 × 0.20%   =  0.80% per year

0.992^40    =  0.725

You keep 72.5%.

Four trades a year is not active#

Most of this room will do four in March.