AFL rookie Paddy Dow impressed in his pre-season debut for Carlton last night but he learned a valuable lesson – always wear your mouthguard.

Dow had an embarrassing moment after his side's 22 point-win over St Kilda, having to explain to viewers why he lost a tooth during the game.

"I was on the boundary and I just wasn't looking… took a shoulder to my mouth and knocked my tooth out," he told Fox Sports.

"Where was the mouthguard?" the interviewer asked.

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"Yeah, it was in my sock," Dow admitted sheepishly.

Despite the missing tooth the No.3 selection from last year's AFL Draft had a strong outing, picking up 13 disposals and booting two goals.