preposition

[chum]

which is correct?
1. "I wanted to cook lunch but thinking that mom will be home by 2, i didn't"

2. "I wanted to cook but on thinking that mom will be home by 2, i didn't"

Is on necessary there?should it be always there?

thanks!

[Rincewind]

Both are correct but the meaning changes slightly with the addition of "on".

1. I am continuously thinking the same thing.

2. I changed my mind when a thought occurred.

[chum]

thanks!