Two things accidentally got mixed in Barney's lab--he was working on some sort of strength supplement when something else got spilled into the mix (without his seeing). When he fed it to the mouse, it had telekinetic powers. Then Barney knocked over the beaker, and it exploded. When he woke up, he had the power himself.
The smoking is in your friend's memory because Barney was also growing pot in his lab (for his friend Peyton). It was the discovery of the marijuana that got him in trouble with the Principal and Miss Burnhardt.
That is what I remember as well from the movie and it has been a long time since I have seen it. The last time I saw it on TV was there is a station out of Chicago called WCIU Channel 26 The U. They had this afternoon show on a Saturday or Sunday afternoon called Svenghoulie. That was the last time I saw Zapped.
That was 4 or 5 years ago when I saw it on TV. They do not show anything like this anymore you know.
Barney got his powers trying to give his mouse some of that elixer and the mouse having kenetic powers pushes Barneys arm into the potion that is thrown onto the floor creating the smoke and the explosion givng Barney his powers.